Transport protocols and their multipath variants used to assume that network paths are symmetric and bidirectional. Actually, network asymmetries (ADSL, satellite) are frequently observed, and some network paths can flow packets in only one direction (e.g., due to a firewall). This article proposes to consider the more generic notion of unidirectional flows. Based on them, we design and implement Multiflow QUIC, a variant of QUIC that is aware of network asymmetries to spread data over multiple network paths. Our evaluation shows that this more generic approach is beneficial in asymmetric cases while being equivalent to multipath approaches in symmetric ones.