Equalising Line

Water will find it's own level without the aid of calculation as long as it's below the minimum water level (it won't do any good if it's at the top of the tanks).
It's the rate of flow through the line wich may be of interest, in which case if the equalising line is the same bore as the outlet, then I don't see there can be a problem. (Although water will generally try to find some way of missbehaving).
If it's a small line T'd into the outlet line it might be subject to venturi (or some other) action which may or may not help the equalisation, or conversely stop it happening while flow is in progress.
If you don't mind a bit of discrepancey in level which can stablise overnight the line can be a smaller bore.
Seeing as you have given us no idea of tank volumes, outlet flow rate and duration etc we can't really be more specific.
Del

the tank operates at 2x50%. the outlet flow is 54 m3/h . volume of each tank is 8.5m3. both inlet and outlet flows are continuous.

I would put it below the outlets, below the lowest level where you need equality.

I would have thought that the system would self equalize, if one tank filled up more quickly or discharged more slowly. the head would climb creating more pressure, and more flow, at the outlet.

Isn't it the same sort of application as one of these:

?

'to conical bottom tanks'

I assume that you mean 2 tanks.The shape of the tanks is irrelative.Put the equalizing line below the low height of the higher level tank.