I am trying to set up a link with two Mikrotik routers and four antennas for maximum throughput in MIMO nv2 configuration.
I searched the forum and found quite a lot of information scattered around but no concise answer.
My question is this:
Using two grid antennas at both ends, should the antennas have the same polarization, or different?

I understand that either configuration works but what are the advantages and disadvantages of each?
The antennas at either end are to be mounted on the same mast, one below the other.

My hardware is the following:
1) Two RB435G with RouteOS 5.24, one at each end (one is running as access point, the other as station)
2) Two DNMA-92 Wistron cards, one at each routerboard. They support 2X2 MIMO and two antennas will be attached to each card.
3) Four Grid antennas 29 dBi at 5150-5850 MHz. Two of these will be attached to each Winstron card.

Software:
RouteOS v5.24, nv2 enabled, two chains enabled.

The best for MiMo is to place your antennas (if 2 of them as you say), one horizontal and one vertical (through if they are seperate antennas and not one MiMo antenna like yours, you could place them as you wish).
Best is one vertical and one horizontal, for MANY reasons (which will take me a LOT of time to write down/explain)
Just put them V and H and that's all.
BEWARE NOT to put other cable (pigtale or so) on V and other on H from your MiMo radio in one place (AP) and the other place (CLE), because you're f(l)ucked...

Now due to 2 SEPERATE antennas with EACH one of them having its own lobe (transmission diagram), try to give at least 1m distance between them, NOT for intefearing reasons betwen them (because of MiMo device you'll work with) but because you "might" "destroy" their transmission diagram (if you can understand what I mean).

Thank you very much hipro5. You have been most helpful.
The reason I am using 2 antennas is that I need the 29 dBi gain, as I am trying to cover 17 km over sea and some hills in between.
Only recently I have seen dual polarization antennas at that gain, and they are not yet available in my country.
I will try for a separation of 1 meter.

As long as one is V and one is H, it doesn't matter which chain is which on each end. Chain 0 on the AP will still communicate with chain 1 on the client if they're crossed up.

It will be easier to keep up with if you choose a way to do it and stick with it, but as long as you have a proper dual polarity setup with an 802.11n radio, it doesn't matter which chain is what polarization.

Why not use a Rocket Dish from ubiquiti and a ubtik or rockettik? You can get a rocket dish up to 36 Db and a mikrotik 711 in an encloser that will snap right onto the dish. Best of both world. And if you want to really pimp the link out you could also toss on some RfAmor. Makes for a nice clean install.
Every backhaul we hang consists of rocketdishs, ubtiks (411/AH with R52Hn), and RfAmor. We have been having great results with this setup. Current longest link running this setup is 28 miles runs about 60 - 80 Mb of traffic with no problem. This summer we are going to replace the 411s with 711s so we can get the gigabit ethernet. After that upgrade I would expect to see upwards of 100Mb.

Yes 0ldman is right as for the MiMo. I had in mind my systems here which have wlan1 and wlan2.

Why not use a Rocket Dish from ubiquiti and a ubtik or rockettik? You can get a rocket dish up to 36 Db and a mikrotik 711 in an encloser that will snap right onto the dish. Best of both world. And if you want to really pimp the link out you could also toss on some RfAmor. Makes for a nice clean install.
Every backhaul we hang consists of rocketdishs, ubtiks (411/AH with R52Hn), and RfAmor. We have been having great results with this setup. Current longest link running this setup is 28 miles runs about 60 - 80 Mb of traffic with no problem. This summer we are going to replace the 411s with 711s so we can get the gigabit ethernet. After that upgrade I would expect to see upwards of 100Mb.

Keep us posted on progress of throughtput increase with board change

Why not use a Rocket Dish from ubiquiti and a ubtik or rockettik? You can get a rocket dish up to 36 Db and a mikrotik 711 in an encloser that will snap right onto the dish. Best of both world. And if you want to really pimp the link out you could also toss on some RfAmor. Makes for a nice clean install.

There are two reasons for not using a RocketDish. The first one is that my house is located at a gulch with very strong winds, and therefore, a solid dish is out of the question. The second reason is, of course, that I don't have a pair of RocketDishes. The question was what to do with my EXISTING equipment to get maximum throughput. I already have 100 Mbps with one pair of antennas, and trying to reach 200 Mbps with two pairs. If I were to consider a no holds barred link, I would go with WDM on a single mode fiber, and get 10 Gbps.

5GHz 30dBi MIMO Dish antenna

@keraia
Using two grids for this purpose I would expect isolation between each grid and other antannas on a mast will have a effect on the performance

The Lanbowen dish mounting brackett is not well designed and will fail during high winds,

Survival Wind Speed - km/h - 200

- the dish may survive but the L shaped brackett could be reduced to a V or the L expanded ?

HA. HA, HA.........200km wind survival with THIS mounting brackett!!!..........GIMME A BREAK MAN!!........
Maybe - as I see it - ~ 50km and I say a LOT!!.......

HA. HA, HA.........200km wind survival with THIS mounting brackett!!!..........GIMME A BREAK MAN!!........
Maybe - as I see it - ~ 50km and I say a LOT!!.......

I would like to see how the test for this was done and for how long or the data spec has a typo = should be 20km/h