If the goal is to have a deeper weapons bay for larger diameter weapons, then a twin engine setup would make sense and twin non AB WS-21/19s would certainly be the most obvious function.
Though I also think if the goal is to maximize weapons bay efficiency for such an aircraft, weapons bay length might be more desirable than weapons bay depth (weapons bay depth is more conducive to rotary bomb bays, or for larger diameter unitary loads, both of which seem a bit ) -- a longer weapons bay of moderate depth would enable the carriage of standoff weapons (which tend to be longer) as well as continuing to carry multiple unitary bombs of normal/regular diameter.
A twin engine design with a single longer weapons bay, similar in planform to X-47A or the CAC UADF might be more desirable, if a twin WS-21/19 design was actually desired imo.
Meanwhile, for a more traditional longer wingspan flying wing, a single WS-10 for pursuit of SFC seems more logical to me.
Speaking of which, I did some rough measurements on the GJ-11/21, which yield a length of ~4.3 meters for the two IWBs by taking the length of the GJ-11 at ~11 meters, i.e. roughly similar to the lengths of the IWBs on the J-20/A/S and J-35/A.
This should enable the GJ-11/21 to carry largely the same missiles as the J-20/A/S and J-35/A inside their respective IWBs - Namely, the PL-15/16 and the rumored-to-be-WIP hypersonic cruise missile (HCM) mentioned by the Guancha Gang sometime ago.
Going forward - I was thinking that since we already have the J-36's larger centerline IWBs, which measures at about 7 meters long (which should enable fitting the PL-17 inside from the outset) for reference - Sooner or latter, there could (if not should) also be a new type of HCM which would make full use of the J-36's centerline IWB.
Therefore, longer IWB(s) on a hypothetical GJ-11-successor UCAV does look to be the natural course of progression/upgrade from the GJ-11. These UCAVs could even act as proper missile trucks for the J-20s/As/Ss and J-35s/As which aren't capable of carrying missiles of such sizes by themselves when conducting A2G and A2A missions against enemy forces at extended ranges - All while meaningfully reducing reliance on non-LO platforms, i.e. J-15Ts, J-16s and H-6Ks/Js/Ns, especially when operating in contested/hostile airspaces.
As for IWB depth - Given the size limitations of the UCAV, a rotary launcher is far from an ideal option. I do wonder if having a missile stack rack would be a viable option here, similar to how the bombs were stacked inside the IWB of an Avro Vulcan:
This could permit the UCAV to carry similar if not slightly larger number of missiles inside just one deeper IWB (for a twin non-AB WS-21/19 configuration).
Last-but-not-least - Needless to say, having two non-AB WS-21s/19s instead of only one non-AB WS-10/15 will result in a larger payload capacity, thanks to the larger amount of thrusts available. A larger-sized UCAV airframe from having two engines could also point towards greater internal volume which can be allocates towards more fuel capacity -> Larger combat range/radius.
But of course, if having only singular non-AB WS-10/15 engine is consisered to be more cost/fuel-efficient is deemed to be one of the key requirements for said UCAV, and that the tradeoff in payload capacity (and perhaps combat range/radius as well) is deemed acceptable, then the single non-AB WS-10/15 engine it is.
