Jacob,
APs used to come in two types: AP (Campus AP for use with controller) and IAP (Instant AP that can cluster together and form a virtual controller together).
As explained before, you cannot mix AP with IAP as the AP needs a physical controller. If you have a physical controller you can use either the AP or convert the IAP, which might cause some confusion; but this appears not the case here.
If you have an IAP-115, you can add another Instant AP to that, given that it runs the same firmware version. Check here to see the maximum version of firmware that can run on an AP. If you had an IAP-135 (with the I in front), it would work. If you buy another IAP-115 (though that seems to be end-of-life), or IAP-215/315 (or 205/205H/305/225/325/335), you should be able to build a cluster. Please ask your Aruba partner if the IAP you want to purchase can run together with the existing IAP-115.