If you wanted the square wave aligned for a split supply amp, then you could use a cap, though you can just sum normally then put your capacitor.
For the aforementioned reason, I'd just use a summing amp instead of a passive sum just because you maintain your capacity to drive some load and assure that the impedance the CD4046s are driving. Simplifies plugging whatever is needed down the chain, and in the event you need some gain, you can do that easily.