the closest people will get is about six feet.
That would mean good 20/20 vision quality at 6 feet, equals 286.48/6 = 47.75 PPI output will suffice, more is better but not absolutely required for image quality (see attachment). To achieve that, you would need to supply an image of 8235x5490 pixels. That would suggest that a single row pano stitch of say 3 portrait orientation tiles with 18% overlap would be preferable as input, because a single 5616x3744 pixel file would give somewhat lower detail quality (may still be adequate but with reduced detail nevertheless).
It also means that the printer driver will be required to apply additional magnification to get to 20 feet wide at 300 PPI (if that is the actual native resolution of the printer), with the possibility to sharpen at that output size after up-sampling. It also means that any sharpening artifacts of the source image would become clearly visible, so optimal capture sharpening without creating halos should be applied at Raw conversion time.
Now, 20x10 feet at 300 PPI equals 54,000x36,000 pixels (1.9 Giga-pixel, ~5.8 Gigabytes), which possibly exceeds the file format size that your lab can handle (e.g. PSB format would be required for a single file). They may be well equipped to do the up-sampling themselves, which would be great if they use good software. In addition, the print will not be printed as a single image in one piece, and it will be easier to transport when mounted on smaller panels (e.g. 10 feet high, and each at printer width), so it's preferable to have the Lab take care of all that, assuming they know how to maintain high quality throughout the resampling/printing process (which also at some stage involves conversion to their output profile, ideally when still in the 45.2 megapixel stage in 16-bit/channel mode, earlier in the process).
Are multiple images spliced difficult to do?
That depends on how the Lab will do the separate sections/panels, printing with or without overlap for mounting and cutting. I'd leave that to them to figure out. They presumably have more experience than you.