The third-party Amalgam monster template consists of two fused creatures.
Unlike other monster templates that are applied to single creatures, the Amalgam template applies to two creatures, and results in one. It has a combination of both creatures' strengths and weaknesses. The Amalgam rules are complicated and awkward, and don't synergize well when the component creatures have class levels or different types of spellcasting. Note that the template doesn't specify how an NPC would perform the fusion.
If you apply this template to Nualia and Lucrecia, you may have to adjust its stats.
There is also the Dichotomous template, which fuses two outsiders, but Lucrecia is a monstrous humanoid.
Fusing enemy NPCs often results in a weaker encounter.
One creature, with the combined stats of two creatures, is generally a weaker fight than the two creatures separately. This is because action economy is valuable, if not the most valuable resource in the game. As a fused creature, they have one turn per round. They could easily be surrounded or flanked, and one unlucky saving throw against a PC's spell could negate their offensive power, if not take them out of fight entirely.
In contrast, if they attacked as a team, they would have two turns per round. Two spells, or two full-round attacks, or one of each. They can also use flank tactics, assist each other, or target different PCs.
What does the lore say?
Mokmurian is evil, but he probably wouldn't try to fuse them into some awkward hybrid. If Nualia and Lucrecia approached him with the intent of getting his help, he would probably
try to assist or recruit them, because he wants to build an army.
You could also modify their stats (such as applying the Advanced template, extra levels, or buffs with permanency) or equipment to make them more powerful, because
Mokmurian is a wealthy and powerful transmuter, with a library of ancient Thassilonian knowledge, and can craft magic rings or scrolls of 7th level wizard spells with caster level 14
With those sort of resources, a Nualia + Lucrecia teamup revenge fight could pose a real challenge to the PCs. Note that the Rise of the Runelords adventure path already suggests what these NPCs would do if they survive against the PCs.