If size matters, I'd think that dual-gang trimpot is a tad large and somewhat expensive at 65 cents (that's more than the cost of all the other components). Presumably this is an infrequent or once-and-done adjustment. That said, why not use two individual tiny low-cost trimpots for R3 and R4. Sure, you have to make two adjustments nominally set to the same value. But this actually can have an advantage in that you can "trim" out any C2 vs. C3 variations to get closer to 50% duty-cycle.
OTOH, if you're only looking for a small trim range, say +/-25%, you could use just one trimpot inserted between the junction of R3-R4 and the positive supply. If the single trimpot value is similar to or preferably smaller than the value of R3-R4, the circuit should behave. For example if R3-R4 are 100K, a 50K trimpot should work fine.
Separately, this circuit should exhibit flash-rate dependence based on the voltage. I notice you have a rather large variation in voltage range. If you are messing around on the bench and have a variable voltage supply, that might be another way to vary timing without using a trimpot(s) in place of or in conjunction with R3-R4. For the curious hard-core DIY'ers, at larger voltages this popular design (it seems to keep popping up) has the interesting property of momentarily turning the 2 transistors into Zener diodes across the reverse-biased E-B junction on each alternating flash. This makes the timing dependent on the DC supply voltage though doing the math would be tedious to say the least!
