Switzerland - Slovenia

As far as the tax authority is concerned, you can not live in two counties, but you have to declare one country as your "nexus" tax state for yourself and your business. That is, the country where you live and where you do business must be declared, and it will be in that declared nexus country where your business and you will be taxed. Note: where your business is taxed can be different from where you are taxed unless you are a sole proprietor.

If you have the capital, and your partner is a Swiss citizen, I recommend setting up a Swiss company with your partner as the Swiss member (required). This will cost you at least 20,000 Swiss francs, but the tax rates in Switzerland for a company are much more business friendly than in most EU states. And having a "Swiss" business address simply looks better on the international market.

If you also declare your personal residence in Switzerland, then you can also be taxed under Swiss law which is probably more favorable than EU nation law (and then you can also start a sole proprietor business which does not require the 20,000 Swiss Franc investment). However, that also means you have to have a Swiss residence permit, and you have to pay all appropriate Swiss social taxes as well (not cheap).