You meet or phone them both and tell them that you do not plan to be exclusive to either of them at this time. Put it like that, exactly.

When they ask why, and they will, you may tell them anything as long as it's the truth. Tell them that you are very new to all this and you want to explore these new feelings and experiences. Tell them that you don't want to be exclusively comitted to someone whom you've only just met. Whatever.

If they are well-adjusted, secure men, they will understand and give you the thumbs up for it. If they are not, then they will become angry, hurt, etc. and let you know in their own individual ways that they don't find your terms acceptable. If that occurs, then drop the not understanding party and continue with the one that understands.