One (large) dive float between 2 spearos diving together sounds fine to me - but I would not use 2 float-lines. Just use one float-line.
And, if you are planning to dive that close together, perhaps it be safer to take it in turns: 1 up & 1 down.
If you shop around, you can buy decent single-skin inflatable dive buoys quite cheaply (around £10-£20, sometimes less) and quite nice big, name brand, "bladder in orange shell" ones for little more. They pack up small, are light and are easy to carry. A lot of new spearos overlook this tried and tested, cheap & cheerful option - which are often used by top competition spearos. As you already have one "boat float", which you are happy to share, why not pick-up a cheap torpedo float for the other diver? That way you might get some of the benefits of both options and a chance to "compare & contrast".
What do plan to use your float for? It will typically server several purposes, such as some of these:
- Warns others (esp. boats) that there are divers present. So it should fly the appropriate Diver flag for your area, with ballast to keep the flag mast upright &, if necessary, wire to keep it unfurled i.e. visible. This is for your safety.
- To attach your float-stringer to, to hold some or all of your catch (alternatives inc. cut-able waist stringer or fish box for sharky waters).
- To carry a small amount of gear (e.g. water/whistle/torch/phone/radio/...).
- As an attachment point for your float-line. At the other end, the float-line might be attached to:
- the butt of your speargun, to avoid loss
- your spear-line via a breakaway-rig, to allow you to detach from large game fish and "play" they via floats, bungees and lines
- a small anchor, lead weight or "kelp-clip" to allow it to used as a marker to diver around
- to a ring on the diver's quick-release weight belt (presumably to help keep track of the down diver and/or aid recovery of the weight belt if it needs to be release)
