You own a store, you sell 2 brands, which?

The real answer is which two brands give me the highest volume of sales as well as profit margins and which companies back up their products. 

Luxman
Klipsch

The OP did not say the goal of the subject matter was to select the two products that would be the most profitable. Suppose you are independently wealthy and opened the store for personal enjoyment.
In that case I would select two products that I would enjoy selling and have confidence in their quality and musical enjoyment. 

Sugden electronics & Sonus Faber speakers...

OK, accepting the premise of the question, and having co-owned a HiFi store back in the day, the brand selection process is not one takes lightly. Leaving aside the Parent Company Ploy that gets you a whole basket of brands (clever, but not the point), picking two brands that can cover a lot of territory and thus give you the best chance of surviving, is actually pretty simple,

Marantz and Monitor Audio. Both are long-time brands with a lot of credibility, both have vertically stacked their product lines e.g. Bronze,Silver, Gold for Monitor Audio. As well they have horizontally diversified their offerings,

Monitor Audio has bookshelf, floor standing, in-wall, on-wall, pre-packaged surround and even soundbars. They have really covered the waterfront, with no apologies necessary at any price point you care to pay, and a couple you probably wouldn't - the  OK, accepting the premise of the question, and having co-owned a HiFi store back in the day, the brand selection process is not one takes lightly. Leaving aside the Parent Company Ploy that gets you a whole basket of brands (clever, but not the point), picking two brands that can cover a lot of territory and thus give you the best chance of surviving, is actually pretty simple,

Marantz and Monitor Audio. Both are long-time brands with a lot of credibility, both have vertically stacked their product lines e.g. Bronze,Silver, Gold for Monitor Audio. As well they have horizontally diversified their offerings,

Monitor Audio has bookshelf, floor standing, in-wall, on-wall, pre-packaged surround and even soundbars. They have really covered the waterfront, with no apologies necessary at any price point you care to pay from under $700 a pair for the Bronze 50s, to the PL500-IIs at a cool $35,000 a pair.

The $699 Marantz NR-1200 is the modern take on the classic Marantz receiver that pretty much defined our hobby with streaming and Bluetooth added as well as phono inputs; the PM7000N, a true middleweight contender and very integrated amplifier, and the muscular Ruby 200W/Ch 4 Ohms, no less than 6 AV Receivers, AV separates, a stellar selection of disk players, and a couple turntables, with the TT-15S1 being a legitimate high-end offering.

So yeah, I could make a pretty good run at it with those two lines, and tell a pretty good story to my customers about the legacies they're buying into.