Could 2 synchronised RX chains be used to receive a signal too wide for just one of them?
In lack of more background, I can only give a general answer:

What you describe is simply an interleaved ADC; that is a technique employed in very high-rate ADCs. And high-rate definitely is more in the range of hundreds of MHz, not a mere handful! For your proposed rates, cheap ADCs can directly sample sufficiently fast. In fact, in the context of SDR, you'll find that most ADCs used are actually doing higher rates on two simultaneously sampled channels in order to allow IQ signals to be meaningfully digitized.

So, yes, that is possible, but it's incredibly hard to do well-synchronized and balanced enough. Just get a single sufficiently fast ADC. In fact, I'm not even sure what SDR-centric ADCs you're looking at that these rates become a concern at all.