Yup.  Don't do that.  Splitter cables are intended to have one driving device (BV *or* BS9000) and two speakers.  A switch box -- common manufacturers are Almando and OneRemote.dk -- will use either the presence of the Powerlink "Power" pin or the audio signal to switch your speakers back and forth.  (There is a technical answer that I just don't remember about how line output impedance of the source and RCA line input impedance of the speaker['s built-in amplifier] are supposed to relate, and what happens if you put another source on that Y in parallel with the speaker, thus the original source "sees" the wrong  impedance, not just a speaker or two.)

P.S.  Don't throw away that splitter cable -- you'll need it because the Powerlink switch boxes only have a single DIN-8 jack output!

P.P.S.  The true B&O-heads here will tell you that you ought to be running a MasterLink cable between those two sources, for an integrated setup.  One remote, speakers connected to one device, automated source switching thru it, routing the IR remote commands -- totally B&O-whole-house-cool!  But the switch boxes work with *any* two sources and don't require learning anything about B&O systems' "Option" settings.  Given that you started with a simple Y cable, I suggested the easier-to-understand method.  But it is sub-optimal if you could get MasterLink configured instead: both your BS9000 and BV9 support it!

That impedance came to my mind but I didn't "believe" it because level

went down even when BS9000 was not powered on at all.

Have to go through boxes if I can find an ML cable. If not might buy one of those boxes you suggested.

Thanks!

Having boxes of B&O cables means you have done this before; good!  But if your boxes leave you empty-handed, as a done-this-before oldster, you might not be familiar with the modern method of connecting MasterLink: buy a pair of cheap ML male -->RJ45 female adapters and run an Ethernet cable between them (preferably shielded) -- much easier to find custom lengths than buying a genuine ML cable.  Cheaper than the PowerLink auto-switch boxes, too!

Thanks for advice. Found ML cable from local dealer;)