We are refurbishing a duplex with solid maple floors throughout the house. I am concerned with the gaps between the planks. We cleaned them out good after remodeling (they were full of plaster). We like the look of the gaps but are distraught about what might happen to the downstairs apartment if liquid was spilt upstairs. Are the 2 coats of highgloss poly, followed by 1 coat of satin poly enough to protect the downstairs ceilings? If not, what would you recommend (wax, a topcoat sealer, etc.)??
Also, should the kitchen and under the appliances be treated differently?

Thank you for any help you can offer.

If the maple flooring was installed over a wood subfloor, and I suspect it was, there will not be a problem with minor spills and such. If you have a very bad water leak, that could seep down through into the unit below. But no floors are truly waterproof anyway. That's what homeowner's insurance is for.