When diagnosed, it is time to have a long chat with the doctor about your options. If it is curable (all bacterial ans some viral diseases). then get it cured with no sexual activity until you get a clean bill of health from the doctor. If it is not curable but is treatable, (many viral diseases), then begin treatment and find out how contagious you may be and determine limits on your sexual behaviour.
There are support groups to which many doctors refer patients to learn more about these diseases from other patients. It is fairly common for people with the same disease (herpes, HIV) to end up as pairs. These are honest relationships and everything is understood by the parties at the beginning.
The last thing you should do is continue to be sexually active putting others at risk. Condom are great but not a perfect solution. They do not protect sores that may be outside the area covered by the condom. And condoms do fail. Do not put others at risk.
A specific answer to a specific question (I think: I had difficulty reading your post) is that two people already infected with HIV are not going to make things worse for each other. Same for genital herpes. There should still be some limits and these should be discussed with the doctor involved. Yes, they could pass things back and forth, your words, but if both are treating, no problem. You would likely avoid sex between partners with genital herpes whilst either was experiencing an outbreak.
Doc is quite correct that there is a lower infection rate of viral diseases through oral-genital contact than genital contact alone. But, it is still a distinct possibility (see his comment about body fluids) and oral transmission of most bacterial infections (think gonorrhea or chlamydia) is just as efficient as genital transmission.
Until and unless you are in a very stable monogamous relationship every sexual contact carries some risk. Please be careful for yourself and for your partners.
Posted: 30 Sep 01:57