If you want to get technical about it... DNI of any kind, whether by Trodes against your skull, or Datajack wired into your grey matter... are responsible for both sending and receiving signals into your noodle so you can experience AR and VR.
If you overlapped 2x sets of Trodes, there's a good chance they would start reading each other instead / as well. Like holding two phones against each other, so the callers are talking to each other. One set of Trodes would try to send you Matrix imagery, and the other set of Trodes would pick that up thinking it was input from you. They'd realize they were both sending and receiving conflicting gibberish data, and probably just Blue Screen.
Same thing would most likely happen if you tried to get multiple Datajacks implanted. They'd both be sending and receiving to the exact same locations in your brain, and when you tried to think commands towards one of them, the other(s) would try and execute too.
I think the easiest solution for "I want to hardline all the things" is to just let your players purchase a USB UDC Hub where they can connect everything, with a single line running to their DNI. Basically an external router.
The difference between this and an Internal Router implant being that the Internal is pretty much just for implants. Letting your Wired Reflexes and your Reaction Enhancers play nice together, without risking either one to wireless hacking. This external UDC Hub would just let you hardline your toys and trinkets together. Neither one will fill in for Wireless Bonuses which require Matrix access though, such as the bonus dice from Smartguns and Medkits.