Hello, I want to build LTE modem that can simultaneously use 2 SIM cards and load balance between them (to make summarize BW of 2 cards and the same time)

For the lab enviroment let's take:
1 modem https://mikrotik.com/product/r11e_lte
1 miniPCI-e modem https://mikrotik.com/product/wap_lte_kit
1 external antenna https://mikrotik.com/product/mant_lte_5o
2 pigtail cable https://mikrotik.com/product/acsmaufl
2 SMA cable https://mikrotik.com/product/smasma

WAN LTE kit already has 1 build in LTE modem and 2(!!!!!) omnidirectional antenas.
So I install into it's miniPCI-e slot another modem, and connect it to external antenna via cables. Is that all? Or maybe I can connect it to one of two internal antennas?
What hardware configuration should I make to use 2 SIMs?

Simply put, you can only have one SIM card active at a time, so there is nothing to load balance across SIM cards. See https://wiki.mikrotik.com/wiki/Dual_SIM_Application for the details on how it works.

Also there is only one modem inside the WAP-LTE and ONE sim slot. The newer wAP R AC, https://mikrotik.com/product/wap_r_ac does support multiple SIM cards, but it also only has one modem. Still only ONE SIM card is active at a time. An external antenna can be wired up using the parts you suggest, but the internal LTE antenna are just not used in that case.

Mikrotik does have models that support multiple miniPCIe LTE modems. I'm guess you may want the LtAP LTE kit instead of the wAP modems...but that has issues with multiple modems at present (it's pretty new). The other alternative would be https://mikrotik.com/product/rbm33g or https://mikrotik.com/product/RB953GS-5HnT-RP which require a case, antennas, etc. – those would work fine with 2 x R11e-LTE modems. There are plethora of examples of using firewall mangle and route tables to do load balances across multiple modems once you actually had the parts elsewhere.