They could be doing something as simple as you're doing, i.e. two static routes. Although instead of using the default, they would normally route to your assigned Internet address block. Additionally, your block of address space is probably injected into dynamic routing protocol within the ISP. The ISP will then either advertise it or a larger address block to the Internet.

PS:

Since you're only using one router with dual links, usually an individual flow won't be able to obtain more than one link's worth of bandwidth. Also, two flows may, or may not, use both links. I.e. One link could be congested while the other isn't. If you're actually connected to a single ISP router too, you might inquire whether you could use something like MPPP.