Another thing that can help is doing the DP doggie style, so you're (more firmly attached) penis is going in her anus and the Feeldoe is going in her vagina (which is easier to insert into).
You're right that the longer ones work better, and while I haven't tried holding the Feeldoe's bulb anally my suspicion is that it is the best of the crop, since it has a symmetrical bulb on it.
If you don't use a harness keep a hand handy. A jockstrap worn backwards might work well, too. Would a SpareParts have sufficient space in the hole to hold the Feeldoe and Jimmyboy's penis at the same time?