Great game! Managed to use all the upgrades and pins and got 7 min with just one clock.
My best is 8:22 (decided that that was good enough), and yeah, one timer is the best. There are two pin upgrades that incentivize only a single timer: one that gives 2400 multi minus 300 per timer (so, 2100) and Jupiter (?) that gives 2000 multi to the sand timer with the highest base time (if you only have one...).