One would only have to 'spend on both chains' if the recipient were demanding multi-chain legal outputs.  This would be an unlikely demand since only long term hodlers who controlled fossil UTXO's would be able to create them.  OTOH, such spends could very well command much higher valuations making Bitcoin Gavincoin as a currency highly non-fungible.

When sidechains or some like solution is well developed they can choose any backing store they like.  As long as Bitcoin is still idling along in the background, and especially if a highly distributed collection of full nodes exist to support it, many sidechains will probably choose Bitcoin as their backing store rather than Gavincoin.  I'll be taking a bet on this eventuality (and earning some spending money) by relinquishing control of a portion of my Gavincoin while maintaining control of it's Bitcoin counterpart.  At least if I can obtain a decent price for my Gavincoin-only spends.

As for 'abstaining from transactions', I've had no trouble doing so for the nearly two-year price slump and am not approaching the end of my patience.  Gavincoin will provoke a battle royal which could take many years to complete and may spawn and ecosystem of perpetual combat.  One way or another, the saying "If you cannot stand the heat, stay out of the kitchen" applies.  Those who are not technically and intellectually agile would be well advised to stick with Wells Fargo and Paypal.

edit: strike