I think the bivalency part comes from 3(c) and 12(3). You can have HP and biomass but only one of them can be accredited for RHI.
Eligibility criteria
3. A plant which is a biomass plant, heat pump or solar thermal plant is eligible for accreditation where it meets the requirements (“the eligibility criteria”) set out in—
(a) regulation
4,
5, or
6 (whichever is applicable to the plant);
(b) regulations
7 to
11;
(c) if the plant is not the first and only plant to provide heat to a property, regulation
12; and
(d) if regulation
13 requires that the heat generated by the plant must be metered, regulation
14.
Requirements where more than one plant provides heat to a property
12. (1) The requirements referred to in regulation 3(c) are that where the plant (“plant A”) provides heat to a property to which any other plant provides heat or has previously provided heat, in relation to that other plant, or if there is more than one in relation to each such plant, (“plant B”),—
(a) paragraph (2) or (3) applies; and
(b) paragraph (4) applies.
(2) This paragraph applies if plant B—
(a) is not, and has not at any time been, an accredited domestic plant; and
(b) is not a plant for which an accreditation application has been made and has not been rejected.
(3) This paragraph applies if plant B is, or has previously been, an accredited domestic plant and—
(a) either plant A or plant B (but not both) is a solar thermal plant; or
(b) plant B is an original plant and plant A is a replacement plant.
(4) This paragraph applies if plant B—
(a) is not, and has not at any time been, an accredited RHI installation; and
(b) is not a plant for which an application for accreditation has been made and not withdrawn by the applicant (and accreditation has not been refused) under the Renewable Heat Incentive Scheme Regulations 2011(
1).
The style of wording of this legislation takes the biscuit. DECC always had the intention for RHI that you could have solar thermal for water heating plus one other technology for space heating but the drafting of this is about as obscure as it gets.