MikroTik

I have:
Link A: SXT1 <----> AP1 in SSID: RRID1 , Frequency: 5500 with nv2 and 20/40MhzMhz HT above
Link B: SXT2 <----> AP2 in SSID: RRID2 , Frequency: 5540 with nv2 and 20/40MhzMhz HT above

If I do a test of A or B separately, I have 80Mb in A or B
If I do both test at the same time, then RX CCQ of link A decrease and speed low from 80 to 35

Why?? they are in different frequencys

Are some devices are on the same site?
If yes, how colse are they to each other?

40MHz separation is not enough if you use 40MHz channel width. That is why you have problems. Transmitters have little bleed into neighbor freq that can cause ccq drop at higher load moments. You need to move your channel further or investigate some additional physical measures to improve noise isolation. SXTs as they come are not perfect for that matter.

SXT1 and AP2 are very near, half meter... and can't move it for now.
I will try uses another frequencys, 5500 - 5600 for example, and I will tell you if it works better
Thank you very much for your help

I tried with 5600, and the problem is solved!
Thanks!