These two Senators,with Lieberman as chair,sheaperded the legislation that created the Dept.of Homeland Security(DHS) and therefore have a Clear conflict of interest in the investigation and should recuse themselves.
Decisions they made,including putting FEMA under the umbrella of DHS,will have to be scrutinized and perhaps critisized to the point of admitting mistakes and politicians in a position like this are required to recuse themselves.
It's that simple.
If the Senate decides to do an investigation these two should take no part in it.