An observation on dispensationalism

I confess to not being very familiar with the dispensational vs. covenant theology debate.  And of course, there are different brands for each.  However, since i've been doing some reading in eschatology as of late, this issue has become fairly relevant.  As I was reading through the book of Exodus yesterday, I came across a text that i'd like to make some observations with:

“YHWH said to Moses and Aaron, “This is the ordinance of the Passover: no foreigner is to eat of it; but every man’s slave purchased with money, after you have circumcised him, then he may eat of it. “A sojourner or a hired servant shall not eat of it. “It is to be eaten in a single house; you are not to bring forth any of the flesh outside of the house, nor are you to break any bone of it. “All the congregation of Israel are to celebrate this. “But if a stranger sojourns with you, and celebrates the Passover to YHWH, let all his males be circumcised, and then let him come near to celebrate it; and he shall be like a native of the land. But no uncircumcised person may eat of it.”

(Exodus 12:43–48)

If I understand dispensationalism correctly, God has two covenant communities with unique promises for each.  But here, we see that gentiles (or foreigners) may be grafted in.  Obviously, these gentiles are not descendants of Abraham or any Israelite.  But through receiving the sign and seal of the covenant (through circumcision), they may now be part of the community and (as far as I can tell) receive all their blessings and participate in all the rituals.  It would seem that if YHWH is to have two covenant communities then the gentiles would have been treated as such, just as dispensationalist treat gentiles in full distinction from Israel today.

Here is where I see a problem.  With the administration of the New Covenant, a new "sign and seal" was administered: regeneration.  This can be seen in texts like Ezekiel 36:26 and others.  So, my question is this: if anyone can be a part of the covenant community (whether its the new or old covenant) through meeting the requirements (circumcision for the old, regeneration for the new), then on what basis can we say that YHWH has two separate covenant groups with different promises?  Furthermore, does the Exodus text not show that racial descent has little to do with being part of the covenant?

It could be the case that i'm off on my observations, due to my unfamiliarity of the issues.  So i'll leave it to the reader to correct me if any need to be made.