Is is possible to use 2 modems with switch w/ 1 A?

The iDirect 5100 specification says 120W for the 48V option (see https://www.idirect.net/~/media/Files/Spec%20Sheets/SPEC_SHEET_iNFINITI_5000_Ser... ). The NJT5018 BUC needs 170W (see https://www.vitacom.com/images/NJT5018_series_rev02.pdf ) so you need an external power supply. If the BUC option is "power via the coax" you will need a bias-tee with say 100V DC block rating to feed the DC power in sideways via the cable.  Make sure the bias-tee can pass frequencies as low as your 10 MHz reference. Maybe get the BUC modified for an external DC power supply option via DC plug.

With such a high power BUC, pay attention to DC voltage drop due to resistance of cables. Peak currents may be higher than the specification power need indicates, as the BUC may use an internal switch mode power supply that takes input current in pulses. ( This especially matters if you are transmitting TDMA, as the BUC DC supply current is supposed to increase instantly when a TDMA burst is transmitted - not applicable to you. Poor power supplies cause the leading edge of the TDMA burst to be distorted)

If you use coax switches pay attention to screening (to avoid transmitting spurious signals from local cell phone etc).  Make sure both modems are powered off before switching occurs to avoid transient damage to modems, LNB, BUC or burned out switch contacts due to arcing.  Plugs and sockets or U links are an alternative to coax switches.

Best regards, Eric.