Add a site selection page before website

A client of mine has just branched out to do business in a second location, and there are differences between what each of their business’ 2 locations can offer.

The main website for the first location is on the main domain, with the second location website being created as a subdomain using multisite.

What I’d like to have when people go to the main domain, is first a page that pops up allowing the visitor to select whether they want to go to the website for location 1 or location 2.

I’m stumped as to how I should do this, I’m sure there’s a fairly easy way!

Thanks in advance for your help.