Which type of surface finish to use for low-temperature SMD soldering?
If you really want to go with your route of using two different solders, consider using lead-free solder instead of the low temperature bismuth solder if you are having trouble with it wetting.
But the better way to do it is to just use the same solder on both sides and rely on surface tension of the molten solder to keep the component on the PCB while it is upside down. I read around and it seems that even QFP-64 will be just fine.
Reflow the bottom first, flip it over, then populate and reflow the top side. That way you are never handling the PCB in a state where the components are upside down but not soldered or with molten solder and can fall off.
You can also use Kapton tape or high temperature reflow tack adhesive (tough to find) to keep the larger components on the board when they are upside down while the top side is reflowing. But the better way is to lay out your PCB so the heavy components are on the top side and the light components are on the bottom side, the light side first.
If you do use tack adhesive on the bottom for larger components, apply the adhesive AFTER it has been reflowed so that the adhesive doesn't interfere with the reflow.
Or, if your hands are steady and the solder paste is sticky enough, you could populate both sides and reflow at once and rely on the paste to keep it stuck on the underside. I wouldn't trust myself with this though.