You could feed the output of the first mixing desk into one input of the second (two if your in stereo). Set the input filters to mid range for that channel.
So for example if you have a desk with 8 I/Ps and a esk with 4 I/Ps you would feed the output of the 8 desk into one of the inputs of the 4 desk and get usable inputs in total.
Of corse if your mixing foldback on each channel this setup could make life more complicated, in thatcase youd need to feed the monitor bus into another input on the second desk and bus to include desk 1 monitor signals into the mix.
Do-able, but personaly I'd probably look at upgrading the desk to something that is big enough to start with, less hassle, especially if your working in live enviroments.
