You probably will end up trolling too fast, even on a single 90. The reason is that your props are probably optimized for top end, say, 19-21" pitch (guess). So, if you are idling at 800 RPM, with 2:1 lower-end gearing, and knocking off 50% for slip, your real-world minimum trolling speed is 4 MPH, probably faster than you want to be going.

A 9.9 kicker usually carries a 9" pitch prop, with more slip, for a minimum trolling speed of maybe 1 MPH.

Regards.

Mike