They should be in slots 2/4 if its just 2 DIMMS (counting away from the socket starting with 1). The ones further away from the socket, with an empty slot between them. But you probably already know that.
The 2600 might struggle a bit with various ram settings depending on the IC, especially with 32GB. Possibly downloading Thaiphoon Burner to see what the ICs are could give some hints. Maybe starting a thread in the memory section for this would be better. I will say to go ahead and try some extra SOC voltage, as the Zen+ need a bit of that to run larger capacities and / or higher speed.