I concur that two 5x7" safelights will likely be inadequate for such a large space. Thomas Duplex lights are nice IF they are in good working order. I had one the buzzed like crazy and drove me crazy. I ended up paying shipping for someone to take it. One or two with a silent ballasts and good filters would light a space that large well. Keep in mind, however, that the Thomas lights rely on a reflective ceiling to distribute the light. If you have dark rafters, etc., then direct lighting might be a better option.
For this latter, look for good used Kodak D utility safelights (or even an older C in good condition) with 10x12" filters. The safelights themselves can be refurbished easily; they're just a metal housing with a bulb socket and a hinged frame for the filter. A little sandpaper, some spray paint and they're as good as new. Finding the filters you need in good condition is the more important part. Many will come with OC filters, which are fine for most B&W papers. I found that I needed red filters for the Foma Variant and had to search a bit to find a couple 1A filters in good condition. There are a lot of other filters in that size out there, but which were designed for special applications. Make sure to get filters that are designed for your application.
Another good alternative is red LED strip lighting. I have a strip in my darkroom that's covered with red graphic-arts tape (rubylith) that works great. Some red LEDs will fog VC papers because they also emit some green; that's why the rubylith tape.
My current darkroom has Kodak bullet safelights with pull chains over work spaces on the counter. These I can turn off if they're not needed or remove the filter for white light on the counter if needed. General illumination is from larger Kodak D 10x12" safelights pointed up at the white ceiling. And, as mentioned, I've got a 4' strip of red LEDs over the shelf above my darkroom sink that illuminates things on the shelf and gives some general lighting, but is shielded from the sink by the shelf.
Whatever you end up choosing, do do a safelight test as outlined by Kodak here:
https://www.kodak.com/uploadedfiles..._motion_products_filter_K4_Safelight_1106.pdf
Best,
Doremus
. If you look at the spectra of a low pressure sodium vapor lamp these safelights make sense. Black and white paper is very blind to the narrow bandwidth, our eyes aren't. I don't know what the bandwidth is for LEDs. I don't think I have more than 75 bucks each in these. I have aquired these over the last 20 years. 
