Any thoughts on the ultimate hacking keyboard as an option? https://ultimatehackingkeyboard.com/
Being able to join it together isn't very useful -- if you can type on a split keyboard, why would you want to join it together?
But, it's a decent option that you can simply buy. Many of the better options are sold as DIY kits, or at least require editing a text file and re-flashing the keyboard to change the layout.
There are reviews of some other keyboards here: http://xahlee.info/kbd/ergonomic_keyboards_index.html (and the DIY options under 'DIY').