Yeah that should be pretty simple, you will have to do the form error handling on your site via javascript though. Then once it checks out it can be submitted to the other site IF the other site isnt solely listening to thier form (which is great coding practice but almost no one does it) so in other words if they say on 2nd page to only get values from form on first page, but like i say most just say "give me value from A form field called name) they do not specify which form, I hope this makes sense I am trying to put it in easy to understand text lol.
If you need this done today PM, depending on the fields and how much validation you want done I imagine your looking at about $20
Cheers
James