Hi Kevin,
Yes it is possible to link them. You need a length of food quality pipe and a non-return valve. Place the indirect water container a bit higher than the direct one, a couple of wooden block will do, put the free end of the pipe into the higher container, suck water through the non-return valve end and as soon as the water flows put it into the direct container. The system should now syphon water from teh indirect into the direct, keeping the levels equal until the indirect is empty, but because it is higher you will still have a small amount in teh direct container. You now have 2 containers to fill!
Alternatively, swap teh pump from one to the other as one empties (no good if you in teh shower when it happens) or if water is available on the pitch go for a direct plumbing system, or continuous supply to the water container via a ball valve.
Hope this makes sense
Alan