Carrying both a spinning and fly rod while wading a flat is probably more of a pain in the butt than it's worth. I tried it myself a few times and, on top of feeling very encumbered by the extra rod, also felt like I wasn't having the courage of my convictions. If fly rodding is what you really want to do on a given day I'd say don't hedge your bets. Stick to it, accepting the fact that some fish will always be out of range.