From the datasheet for the LMS7002M chip used in the LimeSDR-USB “Both transmitters share one PLL and both receivers share another.”
So the dedicated TX PLL can be tuned to any frequency from 30MHz to 3.8MHz. And the dedicated RX PLL can be independently tuned anywhere from 30MHz to 3.8MHz. But the two TX channels share that one TX PLL and the two RX channels share their one independent RX PLL (ref: Transmitter side specs )
It may be clearer if you look at the functional block diagram: https://wiki.myriadrf.org/LimeMicro:LMS7002M_Datasheet#Functional_block_diagram
So both TX channels are tuned to the exact same analogue frequency. And both RX channels are tuned exact same analogue frequency (which can be independent of the TX channels).
The two TX channels (and two RX channels) receive (or provide) their data to the digital TSP (Transceiver Signal Processor) part of the LMS7002M chip. The TSP has four independent NCO’s (Numerically Controlled Oscillators), one for each TX channel and one for each RX channel. And ultimately what that means is that both TX channels must be within the same 61.44MHz of bandwidth and independently of that both RX channels must be within 61.44MHz of bandwidth.
The end result is that with default hardware you can not combine two RX channels for more than 61.44MHz of bandwidth. And you can not use two TX channels for more than 61.44MHz of bandwidth.