Yes, both SIM cards will be able to use both antenna's connector, however, not simultaneously. I.e.:
- Teltonika RUT9 series routers can only have one SIM card active at a time. Second SIM card is only for redundancy (You can configure SIM Switch feature from "Network -> Mobile -> SIM Management" menu)
- One LTE antenna on the router is "Main" (used for data transfer), while another one is "Aux" (only used for error correction). It's effect is mostly noticeable in poor signal strength areas.