By Phosphor, with input from other Ghosts
Last updated: 3/12/23
(This was adapted from a Reddit post I made that was addressed to the host/most frequent fronter of a system, so it comes from that direction - there are likely different/additional considerations for non-hosts dating non-hosts.)
First, think about why you want to date your headmate. An awful lot of hosts (particularly in the tulpamancy community) seem to want to date their headmates because they're convinced that they can't find anyone in the outside world. This is unkind both to them and the headmates in question - it really, really sucks to be the desperation date.
Now, what kind of relationship do you both want? Do you want something long-term and committed? Something casual? What about the possibility of your wants - either yours or your potential partner's - changing as things progress?
And what specific things do you both want from the relationship? For you, since you live mostly in the outside world, you should consider whether the things that you want are material in nature, like having someone who can help with everyday chores or take care of you while you're sick. How will not having these things impact your relationship, and how can you find other ways to fulfill these wants and needs?
For example, your potential partner might not be as invested in the outside world, but they should consider their wants and needs as well. What do they need from you in order to feel loved? Is there anything that they feel is lacking or frustrating right now? Do they feel like they're on even ground with you, or is there an imbalance somewhere? Can you commit to putting time aside for your partner just like you would for an outer relationship? If they want to spend time together in headspace, can you commit to honing your visualization skills; or, if they want to spend time at front, can you commit to stepping back and letting them take the reins?
Related to the earlier: if you both want something longterm and committed, what about the possibility that you, the host, might fall for someone on the outside? You can't just brush this off with "oh, but I _know_ [headmate] is The One for me" - seriously sit down with the question, with your headmate. If you're planning to be monogamous, will you feel tempted to end the relationship with them in favor of the outside one? Why? Even if you're both polyamorous, there's things you should still discuss, like whether any outside people you date should be aware of your plurality and internal relationships, and whether your headmate is also interested in outside relationships.
What will be the consequences of breaking up? Unlike outer relationships, you can't just part ways after a breakup - you're stuck in the world's tightest get-along shirt. Would the two of you be able to handle it amicably? Would your headmate have people to help them process their feelings if they need it? It's also possible that you, the host, have significantly more power over the body and even your internal space. How is that power imbalance going to be addressed in your relationship, especially if things sour?
Internal relationships can be rewarding, and even full of wonderful experiences that outer ones can only dream of, but they should be given as much thought as any outer relationship. They are not free of challenges or conflict, and to top it off, they exist in a space that's stigmatized and resource-sparce. They are not a "lite" version of outer relationships, but something all their own.