Is it at all possible to use both simcard slots in the Microtik SXT LTE routers, with 2 different cellular providers?

No.

I think yes, but only one can be active since there's only one modem. Selected under "system / routerboard / modem / sim-slot".

I've seen scripts for failover but it's not something I've tried.

I think the right answer is not, because the right question mean "at same time",
because if you can insert two sim at the same time (on two different slots), is obvious that can be from different providers.

As have to use a script to change the slot used, nothing prevents from changing the APN as well (if needed).

I have an SXT LTE6 that I use as a field test unit. I have 2 SIMs inserted to test the best coverage between ISPs from a clients location. Fortunately, both ISPs (OneNZ & Spark) use the same APN so all I need to do is manually switch between SIMs using Winbox. Seems to work fine.

But as others have already alluded to, if the intent is to run both SIMs at the same time, forget it - at least for the foreseeable future.

But as others have already alluded to, if the intent is to run both SIMs at the same time, forget it - at least for the foreseeable future.

What a pity. I wanted to use 2 different SIMs, so I could route cheaper whatsapp traffic over Provider1 and more expensive general internet over ProviderB

In South Africa, one of the mobile operators have a good deal on whatsapp traffic but their other traffic is rather expensive.

If you want to use 2 Sims at the same time, you need 2 modems.
There is only one included.

You can select the sim which is being used but you can not use both at the same time.

If you want to use 2 Sims at the same time, you need 2 modems.
There is only one included.

You can select the sim which is being used but you can not use both at the same time.

Thank you for the explanation.