I agree with Tomaz. I would not connect the tanks. One nice feature of two seperate tanks is the knowledge that when one runs empty you know precisely what you have left.
If you choose to keep them seperate be aware that the fuel return line on your Yanmar has to be switched from tank to tank at the same time as switching the supply or you will end up over-filling one tank at the same time as emptying the other.
Also running both tanks down at the same time means two places to gather condensation while the tanks are less than full. Your return fuel will be warm and this promotes condensation. You should try to avoid this in both tanks.
The other (perhaps off topic) is your fuel consumption. Maybe you're propped differently or maybe you motor really fast (full throttle?) because I get about 5.5 knots at under 2000 RPM (same engine) and my consumption is consistently around 5 litres an hour. 15l/h would scare me.
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"Two possibilities exist: Either we are alone in the Universe or we are not. Both are equally terrifying."
Arthur C. Clarke