Heck, if they sound like they are wanting to get rid of the boa, see if you can score some kind of deal on the boa and then just one cage. A 36x23x14 isnt going to last all that long with a boa but it will let you try out the Proline to see how you like them plus give you a temporary cage for the new boa. if you end up liking the Porlines and decide you want them, you can order them from Ed at Constrictors NW. But personally, unless you are getting a screaming deal on them, 36x23x14 is more of something for BPs than boas. They will work for juvenile boas but anything bigger, it's not going to be enough length.
And if you end up not liking it, then you can resell it and recoup your cash, especially if you get it for a screaming deal or just keep it for a quarantine cage for any new snakes you plan to get in the future.