In a word, No.

DL can put an entry in the PNR notes of each of the two bookings which references the other PNR. But, that will only be seen if someone actually looks for and acts on it. Agents will tell you that this "protects" you but it does not. It won't hurt, so once you have the four tickets issued into two PNR's, call DL and ask to have the entries made.

If there are flight changes, either in advance or in IRROPS, just make a point of calling in or speaking with an agent. Also, don't forget that even if all four tickets were on a single PNR, there might be occasions when it is necessary or desireable to split up rather than wait for four seats to open up.