On Wednesday, the UUP announced the names of two new MLAs, Adrian Cochrane-Watson and Neil Somerville.
They are replacing the party's two new MPs, Danny Kinahan and Tom Elliott.
On Thursday night, the SDLP will pick a replacement MLA for its party leader.
Dr Alasdair McDonnell is resigning from the assembly on Sunday to concentrate on his role as an MP.
It is thought the Belfast councillor Claire Hanna will get his job as a South Belfast MLA.
Stormont's co-option system was introduced because holding a by-election could change the party political balance within a six-member constituency.
That would happen if an MLA belonging to a smaller party within a particular constituency stepped down, and a subsequent by-election was won by the dominant party within that seat.
The system also avoids the expense of by-elections.
Current co-optees include Sinn Féin's Rosaleen McCorley, Chris Hazzard, Bronwyn McGahan, Maeve McLaughlin and Ian Milne.
The party also used the system to bring Megan Fearon, Máirtín Ó Muilleoir, Declan McAleer, Conor Murphy and Alex Maskey to the assembly.
The SDLP have co-opted Sean Rogers and Fearghal McKinney, the DUP have done the same with Gary Middleton, while the late independent MLA David McClarty was succeeded by Clare Sugden.
Sometimes co-opted MLAs are referred to as unelected politicians, but that is now becoming less straightforward.
Sinn Féin's Conor Murphy was elected as an MLA before handing his seat over to Megan Fearon.
Now he has been co-opted back in to the assembly to succeed Mickey Brady, who succeeded him as Newry and Armagh MP.
Similarly, Alex Maskey was originally elected in South Belfast, but now sits as a co-opted West Belfast MLA.
When Willie Hay, now Lord Hay, stepped down as an MLA and Stormont speaker, the DUP first replaced him with Maurice Devenney.
But shortly afterwards, they had to make a double co-option, with Gary Middleton stepping in to Mr Devenney's shoes.
Once the new Ulster Unionist and SDLP MLAs take their places in the Stormont chamber, 17 of the 108 MLAs will have benefited from the co-option process, which avoids the need to hold by-elections.