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

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. 

No store. With many of the best brands moving to direct sales currently, "stores" are doomed and this is another irrelevant question. Stores like Quintessence are more like museums.

Having been involved in businesses that sell to the public in some manner since I was about 9 years old...

The point is that the basic premise of the OP, is a no show, period. Full Stop.

Sales to the general public, in audio, via a ’store’....does not work that way.

All scenarios are different, all scenarios change. If the store owner does not grasp that and grasp it correctly, and move with it correctly...then they die on the vine.

It will be tough sledding regardless.

Eg, in older times, an ’entrepreneur’ was considered to be about 50 years old. A point where a person was considered to be matured, tested, and varied enough to be trusted with the complex act of being an entrepreneur. To run, or head -a complex attempt. Nowadays... we somehow confuse this with 20 year olds, as if it is all gumption and running in place, expending energy. Walking down the hill (so to speak) is the best way to get it done. Still, to this day.

Finding the right level and quality of maturity in one’s self or another, is more the problem. Eg, you want to open a music shop, a record shop?... you want Richard Branson to run it. He has no more mistakes to make. At the same time he is learned enough to know he still will make them. Which begets the ability to correct with his vast knowledge base of how things can go wrong, a base of knowledge built out of massive levels of scar tissue.

To return the point, the OP is fundamentally flawed. It is a poser that is relatively meaningless and has no relation to the working functional world of such endeavors.