Double docking is sort of a necessity especially on the Danube river. Docking space is rare and therefore the ships - and each year we get more and more - need to dock double or triple.
There´s no specific company which is known for not doing it. The docking space is designated by the harbor authorities. The ships have to dock where the are said to.
Of course usually ships of the same company are docking together. But I have seen "mixed" doubles too.
Regarding water levels... no one can tell you anything for sure unless you have a reliable longterm weather forecast
! You can´t even rely on spring floods in March or April. We had a couple of extreme dry years and low rivers in April til June. We had high water in August for several years which should be low water season. River levels are part of river cruising.