Ok, I made some progress with the BepInEx IO_ExSlider.Plugin to get a futa.
First you need to enable the 3P in Free Mode. Check image 1 for the options to enable it and save the settings (button at the bottom left of the plugin options window), then you need to go back to the Title Screen (for the assets to load) and then go into Free Mode again, which will have 3P enabled this time. You can swap the girls by using the buttons labeled as A in image 2.
Then you can use button B to link the 2nd girl's bones with the MC, so whatever movement he does, she will do as well. They will show up overlapping like in image 2, but you can either displace him with the position settings from the plugin or hide him with the normal game settings (from the ... button at the top right).
Be aware that because the character bones are different, some inconsistencies may show up (like in image 2, the 2nd girl's hand passes through the 1st one's), so you may need to adjust the bones offset in the plugin's setting.