Multichannel RF transmitter design for 315MHz and 433MHz
The MICRF112 uses relatively low frequency crystals (9.84 and 13.56MHz), so it should be possible to switch between two crystals using an analog multiplexer or even a mechanical switch. However you also need something to switch the RF output between two separate antenna circuits, so you won't be saving much (if anything) in BOM.

Considering the low cost and small size of the MICRF112, it might be better to just use two of them - each hard-wired to its own crystal and antenna matching circuit. You could tie the data inputs together and disable the unused IC via its EN input.