I'm not much of a help here but some time ago, I bought these two way manual valves that does this exact thing. Sure wish I could remember where I got them from! I'll keep looking for an invoice if you're interested.
On another note, and perhaps you've already thought of this. If you have a pump that's on demand (shuts off if the water stops flowing), then couldn't you put both the sink and the shower on the same pressurized hose? If you open the sink faucet or the shower wand, the pump comes on.
On our Kamparoo, I heat up the water in recirc mode using the two way valve I mentioned previously. Once the water is hot/warm, turn the valve so that both the sink and the shower lines are pressurized. We get warm water from either sources and the pump does its thing automatically. Just another data point.