You SHOULD use both of them at the same time. HTTP/2 intertwines your data by using different streams, except all of that is on top of TCP and one stream blocking (because a packet was dropped for example) will block the other streams. This is called head-of-line blocking and you basically can't do much to fix this in TCP unless you create multiple TCP connections for your different resources. But this has a bunch of other issues, you will need to figure out congestion control for each separate TCP connection even if they're connecting to the same service :/
QUIC also has multiple streams, but it doesn't have this head of line problem and other streams will continue to work fine if one stream blocks. So QUIC was made for HTTP/2, where your HTTP/2 streams are basically replaced by QUIC streams which are not affected by loss of packets at all.
If you want to see how loss of packets affect an HTTP/2 connection over TCP, it's pretty bad. In general if you're on a lossy connection you should just use HTTP/1. Check this awesome talk: https://www.youtube.com/watch?v=0yzJAKknE_k
QUIC also has multiple streams, but it doesn't have this head of line problem and other streams will continue to work fine if one stream blocks. So QUIC was made for HTTP/2, where your HTTP/2 streams are basically replaced by QUIC streams which are not affected by loss of packets at all.
If you want to see how loss of packets affect an HTTP/2 connection over TCP, it's pretty bad. In general if you're on a lossy connection you should just use HTTP/1. Check this awesome talk: https://www.youtube.com/watch?v=0yzJAKknE_k