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Monolights would solve the power and recycling problem. In addition to what you've looked at already check out the Alien Bees line. AB400 (160WS) are $225 ea, and AB800 (320WS) are $280ea. I use a set of four AB800s. The have built-in optical slaves which means only one radio trigger set is needed. Trigger the one light via radio and it's light will trigger the others. Buff also makes a $100 wired controller I use that allows me to adjust power on the lights remotely from camera position. I plug my Pocket Wizard into it, which triggers them all.
In terms of overall plan you might also want to reconsider using shoot-through umbrellas when you get the second flash. With one light there's an advantage to having the 50% if the light not hitting the subject bouncing off ceiling and walls as "spill fill". That's what is making the shadows lighter and the overall lighting ratio "softer" but you can't control the shadow tone and mood.
Adding the second flash in the role of fill (coming from the direction of camera where it can reach all shadows the off axis "key" light is creating) allows independent control of fill and the lighitng ratio by changing fill intensity. But here's the Catch-22: to get shadows any darker than what they already are with all that "spill fill" from your single shoot-through you will need to use a key light modifer that doesn't spill as much light around the room: a soft box
For example I typically use a med. 24x36 SB on my key light so I can control where it goes. Used alone the shadows are darker than what looks normal in the photos. But by simply adjusting fill based on the detail in the shadows the key light isn't hitting I make them normal looking (3:1 H:S ratio - key 2x brighter than fill), lighter than normal (2:1 - key = fill) for a happier mood and to minimize wrinkles, or darker than normal (4:1 - key 3x greater than fill) which creates an impression of thoughtfulness, sadness, aggression, etc. The skin tone in the highlights of the face are the same with all those ratios. What changes the mood and appearance of the wrinkles and nose it the tone of the core shadows changed by modulating the fill intensity.
1:1 Even fill from direction of camera hits entire subject evenly creating light but flat foundation
1:0 Key light overlaps the foundation shadow detail created with the fill to create the highlights
The resulting overlap of the equal strength lights (incident) results in 2x more reflected light from highlight vs, shadows creating the 2:1 ratio under the portrait ratio convention. A 2:1 is a bit lighter / softer than average room lighting (i.e. baseline normal) which is typically closer to 3:1. In other words if you shot the same face with the same lights just changing the lighting ratio from 2:1 to 6:1 the 3:1 would seem most average / normal.
For darker shadows you'd simply start with less fill and make the shadows darker before overlapping the key light. The problem with shoot-thorough umbrellas is you can't easily reduce the spill fill they bounce around. For better control of spill and lighting ratios, especially on dark backgrounds, you'd want to use umbrellas with black covers in normal reflected mode, or a soft box.
I mention the issues above you might not have considered because the advantage of studio lights vs. speedlights for studio work is that they are designed to make it easy to mount on stands and a wide variety of modifiers via the "speedring" mount: metal reflectors you can use alone or with grids to create controlled accent highlights, any size / shape SB, metal "beauty" dish. etc. That in term allows creating a wide variety of looks in the lighting vs. the lighting always looking like and overcast day; what you'll get with two shoot through umbrellas in a small space.
Yes speedlights can be used, but if your goal is static portraiture with both lights on stands there aren't any advantages doing it with speed lights. Years ago speedlights were needed for location shooting and were cheaper than available studio gear, but lights like the AB line and the Vagabond battery / inverter developed by the same US manufacturer (who has great customer service) changed that paradigm. Now it's cheaper to buy a pair of AB400 than a top of line system flash. Hot shoe flashes still have advantages for location PJ style shooting, but not for static studio tasks.
I know this because I used speedlights (and daylight) for everything for 30+ years before having the space for a permanent studio and buying my AB800s and modifiers. When I started wanting to do more static portraits the lack of power, slow recycle, and limited modification options of speedlights became a logistical and creative roadblock so I bought the studio gear. I still have a pair of speedlights for location PJ style shooting.