'Twould be a nightmare
It's one thing to have two water tanks in tandem...water is being drawn OUT of the tanks, so no chance of overflowing one if you forget to pay attention to the level. But two holding tanks in tandem is a disaster waiting to happen. Plus, it would double all the tank odor control and vent maintenance. The answer is either a larger tank or less flush water consumption, or both. If your head is aft, the v-berth would not be a good location...the hose from the head to the tank would be WAY too long. However, if your head is forward and there's room under the v-berth for a significantly larger tank than the one you have now, that's a good place for one. Check out the Ronco catalog (link below)...they have several bow-shaped tanks. Decreasing the volume of flush water per flush can dramatically increase the number of flushes any tank will hold...what toilet do you have?