I only do two hours at a time. It can be a real problem if you don't do it right.
Basically with one hour, one shot (or two in quick succession), you can just walk in and masturbate with their body, and walk out. With two hours, you'll need to cooldown and recharge. You'll want them to relax you and help calm your body down, and then you'll want them to bring you back up by flirting with you and teasing you. That means she has to be enjoying the experience too. In other words, you two  have to interact as people, as regular human beings on a kind of special date.
Doing a duo helps a lot with that, the girls talk to each other, make it much more relaxed. The girls can also warm each other up, and they can trade-off on you. And with two girls, both more relaxed, its much more likely that one of them will connect with you just naturally.
If its just one girl and your first time with her for two hours, don't fall into the trap of walking in and running to the shower, and then starting. If you can, when orgainzing the session send them an extra $50 - $100, and ask for some flirty emails beforehand. You'll have some rapport beforehand so you're not just two awkward strangers, and it also gives you the chance to tell them exactly what you want, and for them to build some anticipation. Also, spend the first 5 - 10 minutes chatting. Even if the conversation isn't great, it will make the session better. So many things are YMMV, you'd be surprised how much better your time is if you have some rapport and she knows what you have been fantasizing about.