ZCT's secondary connected in parallel to protection relay

The CT will have a relatively high impedance with regards to each other. The relay will have a very low impedance relative to the CTs.
Virtually all the current will flow through the relay element.
A similar application in that it depends on the high impedance ratio between the CTs and the meter element is an accepted connection for metering three phase energy with a two element meter.
"A" phase is metered by a CT connected across the "A" phase element,
"C" phase is metered by a CT connected across the "C" phase element,
The Third CT is connected across the other CTs in delta so that "B" phase current is forced through both the "A" phase and "C" phase elements of the meter.
The "A" phase element meters the "A" phase component of "B" phase and the "C" phase element meters the "C" phase component of "B" phase.
This is significant in that it illustrates that the very high ration of CT impedance to meter element impedance delivers metering quality results.
I have used this connection successfully.

Bill
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"Why not the best?"
Jimmy Carter

If you never have to worry about saturation, then I think Bill's answer pretty much sums it up.

But if you can get unequal saturation, I find myself just a bit leery. Generally you'd use a ZCT when the expected ground fault current is much less than the expected phase fault current; so you get things like 1200:5 phase CTs and a 50:5 ZCT. Now say that you have a high resistance connection at one end of the upper 'R' conductor so that the current split between the sets is not identical. Both CTs will have some standing current, but it will cancel at the summation point. Now, have a phase-phase fault somewhere downstream and if the CT that has too much of the 'R' current saturates your relay may see current that it shouldn't. It might trip on ground for a fault not involving ground one or more zones away. Maybe you can get away with it, maybe you can't .

I'd run this by the relay supplier. These ZCT and GF relays are often customized combinations and not really standard CTs and protective relays.

Make sure both CT's are orientated the same way and correctly aligned in the correct direction.