In order to use two separate containers for sump and fuge you will have to add another overflow. The pumps simply wont even out flow for you. This also means the sump will have to be somewhere around 14 inches lower than the fuge to provide room for a hang on OF.
Here is how I would do it if using two separate containers. I assume the 1200 GPH overflow has two one inch drains. This is the common configuration. Direct/plump one drain directly to the fuge and one to the sump. Hang an additional 600 GPH overflow on the fuge that also drains to the sump. Here again your fuge will have to be positioned higher than the sump to allow for hanging on the additional overflow on the fuge and drainage to the sump. You may not have room for all of this especially if the DT is not high enough off the ground. Water runs down hill only.
The other alternative is one larger container with baffles glued in. This limits ability to change things around at a later date if you wish to change the configuration of the compartments.
I prefer to just have as large and open a sump as room allows. You are then free to change things around simply by slideing different size containers down into the sump for a skimmer compartment, fuge, filtration etc. and overflow plumbing can be changed to direct water where ever and whenever you think is needed. Only problem is your going to need plenty of room behind the DT for this type configuration. Probably the best part of the one seen below is any plumbing drips or drissles fall right back into the system makeing it worry free for the most part
