It seems unlikely that the intention of H and W is for separate funds to be created each held on separate trusts. In which case a single settlement will have most probably been created. The death of H or W will not affect the income tax treatment of the income of the trust.
ITTOIA ss 644 and 645 specifically deal with the issue of two (or more) settlors but only for the purposes of Chapter 5 Part 5 ie “settlor interested” trusts which is not relevant here. Similarly TCGA 1992 ss 86 and 87 (non-resident trusts) are not in point.
However, there may be two separate settlements for IHT (IHTA 1984 s44).