I have a client that has 2 businesses that want to use the outbound numbers for each business respectively. I am not sure if this is possible using SIP trunking ? Am i wrong or can it be done ? they are currently ussing pots line so the can pick which lines to call out on for each business.

It probably depends which phone system the SIP trunks are connected to. Avaya IP Office, for example, can dictate alternate Caller ID for outbound calls simply by using a different outdial prefix (8 instead of 9 or something like that).