The Pre2 does not do this. If the long messages you are sending appear as multiple short messages on the recipient's phone it can be because of 2 reasons:
1. The recipient has an old phone which cannot combine multiple messages into one message.
2. You are using a phone network which does not support this feature.
Very rarely you may have an incoming message which may split into two on receipt or the same thing may happen to your outgoing message but usually this happens due to network congestion where a long multiple message string gets interrupted to allow other traffic and appears on the receiving end as separate messages with differing time stamps.
Otherwise I have never seen this as an issue on the PalmOS (Treo 170, 270, 600, 650 & 680) phones as well as the webOS phones (Pre - & Pre 2) that I have used to date.