Köln/Cologne (passed by there yesterday-with big problems on trip from there) is in the VRS (Rhein/Sieg) Verbund, whereas Düsseldorf is in the VRR= Rhein/Ruhr. So to visit the 2 in 1 day means a ticket for a larger area- in this case NRW=the Land=province, only valid from 9.00 on mo-fr.
It depends if you really want to save the very last €, are willing to take more troubles (used to those over complicated german ticket machines?) or rather go for take it easy. And also if you are with someone else and will travel all trips together.
That DE-ticket (I use it since march about 3-4 x a month from mo-pla as its the easiest to passby the otherwise creditrating which germans are unable to do in general for foreign bankaccounts) and did not note any real tightening of conditions at all) is strictly personal=that means x2 with a partner. It is simply a one price for the whole month and is NOT capped or the like. One can even still start it for the current month on mo.pla whilst that is underway-other providers also want you to decide that before the 10th for the month coming up. Mo.pla as I noted on my last trip is in fact also used by a few much smaller east-german local systems who avoid then a costly set-up to maintain their own app and all that. I have no complaints about it -other as the ARRIVA gimble app which I also have used before. Just show ticket on fone (QR-code) when asked for or in some areas when boarding buses-Not in Köln- I had just 1 check yesterday for using 6 trains and 2 buses (as bypass for a city where all trains were cancelled) for some 500+ KMs.
It used to be that the HVV=Hamburg Verbund had a lower price for latter parts of the month IF very smartly ordered and cancelled, but from some reports I gather this has about ceased now.
Daytickets mostly have a much smaller add-on for the 2nd/3d etc. person, but as I just checked NOT in VRS only! The price will also vary with area paid for-Köln city is 9€ for 1 person, 24 hrs, incl.airport, or 18,20 for small groups till max 5 persons-so no saving for just 2 persons.
There is also-for fone-fanatics, a system NRW wide called eezy.nrw that charges by distance for all trips which you activate in your app on that fone. That may have a cap for the total but again its personal and no lower price for partner. I am not very familiar with such systems, so not info from me.
So: in short: for just 4-5 days and for half of that IN the city, a DE-ticket is not cost-effective, but does save a lot of possible complications and searches.
PS-if looking for hotels with an GB-style brekkie: Premier hotels are now also in Germany, I saw one in Köln too yesterday, but in general Kóln is not a cheap city for hotels.
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