You may have a smallish back garden like me; 10m deep and 14m wide. Your 40m loop (that’s really 40m [or so] all the way around) will resonate on the 40, 20, 15 and 10m bands. Mine is only about 20 feet off the ground, around gutter height. Its not quite square but the far side and the near side are completely different lengths to make it nearly a triangle. Not quite. I have coax to the feedpoint and a cheap 4:1 balun there. With some trimming on a sunny day, you can make it work on all the amateuir bands – and I even used it on 2m once!
My 80m loop was (and still is) square at about 15m per side which is actually too small for the 80m band. My 80m loop is actually 60m all the way around. To make it “bigger”, I added 4 x loading coils into each corner. These coils are 2 inch in diameter, 6 inches long and approximately 30 turns on each coil. With some farting about, you can easily make it tune either the CW or the SSB portion of the band. If you have a tuner, you’ll dial that out easily anyway, particularly at lower power: 400 watts and under. The 80m loop happens to work on 30m band too. Just a fluke. Great for digital.
There doesn’t seem to be much interaction between the two antennas, although when my 40m loop came down in the wind recently, my 80m loop started resonating 200Khz further up the band, towards the 75m band – so there is interaction I suppose, your job is to dial it out.
Living in a housing estate doesn’t mean you have to give up the hobby. Loops are great and you can make them work much better then dipoles (IMHO) for the same amount of real estate. Did I mention copper wire? Sure make it out of copper, but you could try Cat 5 cable, you don’t need special wire to make antennas work.