Splitting Bandwidth using 2 SAT modem

Receive:

Assuming that just one LNB is required, you will need a 3 dB splitter, similar to this:


In this example the antenna LNB is connected to the input at the top.  The two modems are connected to the output connectors at the bottom.  One modem is connected to the power pass side and this modem will supply the LNB with its DC supply voltage.  The power source in the other modem may be switched off, as it is not used.  Some splitters have power pass capability on both sides and have diodes to stop one modem affecting the other.

Note that the splitter does not noticeably degrade the receive signal quality.  The noise level and the signal level are both reduced together, just like having an extra long cable.

Transmit:

You don't say if this is required.   If this is required I suggest two separate antennas and two BUCs.

The use of a single high power BUC will be expensive since the power rating of the BUC will need to be about 3 times the composite power of the two carriers.  So if each system on its own would need a 2 watt BUC then the new BUC to cope with two simultaneous carriers would need to be a 12 watt BUC and specified by the manufacturer as suitable for multicarrier operation, otherwise serious intermodulation interference will occur.  
See here for 12, 16 and 20 Watt Ku band booster amplifiers Note you may need an additional very large power supply for the BUC power booster.

Best regards, Eric.