Why do you absolutely want *.brd files ? What do you mean to do? that sounds a bit weird to me.
Seems you did not looked the wiki or you would have found this for example : http://wiki.midibox.org/doku.php?id=mb-twinsid
You can find layouts for SID, PIC8 Core, DIN, DOUT and so on on http://www.ucapps.de/. With these files you’re welcome to make your own PCBs for personal use, and they’ve been there for at least a decade.
Maybe you want to clone the MBSID for commercial reasons, maybe you want to just etch your own boards at home. But the easy answer for you is: no one is going to give you design files for sammichSID or a complete project like it. But if you’re interested and have skills to make boards, maybe you can come up with a similar project. I’m sure there’ll be many people interested :).
“MIDIbox SID 8580 / 6581 + CS” is the SID kit set on Mike’s site that you want. I think he still provides PCBs and PICs, possibly other parts but I don’t know.
You can also get PCBs and source your own parts. Where you buy from depends on your location. If in the US or nearby try http://midibox-shop.com/
Note you have to build your own control surface linking the DIN (digital inputs = switches and encoders) and DOUT (digital outputs = LEDs). It’s more of a classic MIDIbox this way. You can choose how complicated to build based on your skills, time etc.
I’ve never built a SID module (just the MB-6582), but the MBHP module only takes one chip, so yes you need an additional one if you want a stereo pair controlled by one Core8. Just write to Mike and see how he can help with PCBs.
For your own solution, I’d say that would be possible and you would get some interest. You just have to follow the schematics on uCapps and combine the modules rather than interconnecting them with cables.
Before selling PCBs, you must prove the concept first. But with a prototyping batch (e.g. 10 boards) it’s generally okay to sell the extras that you don’t need.
Honestly, if the idea of building a MIDIbox from scratch seems daunting, you’re probably better off with one of the other options out there.
Look into buying a Commodore64 and getting an MSSIAH cartridge for it. This is a MIDI-controlled SID synth that runs directly on the C64 hardware. If you want some DIY experience, there are plenty of C64 mods you can add to make it more like a musical instrument. Analog pots, stereo SID chips, etc. This kind of set up is nearly as awesome as a MIDIbox, and comes with the cachet of running on the actual C64 hardware.