[BenMcr]

To answer your question straight away, no it is not possible to split the Virgin connection. Virgin also only supply one modem per property (and you cannot buy them legally from anywhere)

What would would have to do is split the connection past the ethernet socket on the back on the modem.

If the Airport Express has a wired connection on it, you could run a ethernet cable to the garage and put a Wireless access point on the end.

If not you are going to have to use another type of wireless router