@CTF, both CPEs are transmitting and receiving. They both need to send data to each other. If your question is which one should run in AP mode and which one in Client mode, the answer is: it doesn't matter for a Poin-to-Point link.

However, the AP transmits and receives and so does the client. WiFi devices are not radio stations which will transmit only one-way from the sender to many receivers.

Since you will have to align the antennas of both CPEs, you need to climb on the mast anyway, certainly many times if you place the CPE610 on the mast b/c it has a very narrow antenna beam width of a few degrees. If you place the CPE520 on the mast, antenna alignment will become easier for this CPE since it has a 45º antenna beam width.

BTW: A 12m high antenna mast must be anchored (secured)! If it is not, it will blow him in the lightest storm. Thus, you have to climb to the mast for anchoring, just take the CPE with you for antenna alignment.