In effect, T-Mobile and Sprint are sharing spectrum and that increases capacity. Large spectrum blocks are less likely to be congested, partly a result of queueing theory. Google Fi uses both Sprint & T-Mobile - and lots of Wi-Fi - choosing whichever has capacity when the phone needs it. With Sprint itself, when capacity is used up the data doesn't get through, even when there is spare capacity on T-Mobile. There's no way in practice to hand over the connection. The same, of course, applies to T-Mobile and every other network.
Until now. When Sprint is congested but T-Mobile isn't, the connection automatically switches to the T-Mobile network and gets through. The two networks have different user bases, peak areas, and physical networks. The peaks are often at different times.
There's no reason Sprint and T-Mobile couldn't do the same with all their customers. The result would be the same as adding spectrum: with cooperation and sharing, you can handle more traffic.