Half speed internal hub gear?
The closest to that is the Sachs DD3 "dual drive" hubs that are a 3 speed IGH with a standard cassette mount, available in 8,9 or 10 speed cassette versions. According to this site the ratios in the hub are 0.73,1,1.36 giving 1.86 between high and low gears. That's wider than a double chainring setup but narrower than most triples (a 20/30/40T triple has a ratio of 2.0 for example).

Those apparently weigh less than a triple crank and derailleur, but since you want that as well as a double or triple you will be adding the weight, as well as the cost and complexity rather than substituting it. Sheldon Brown has done this with the older 3x7 version and seemed to enjoy it.

The issue is the size or complexity of the shifts. Rather than just having two overlapping gears from two chainrings, you now have 6 or 9 overlapping gears in much of the range (so there's lots of ways to get a given gear ratio). When shifting from high to low in the hub you either experience a big jump in final ratio or have to shift significantly on your other levers at the same time. If you went from, say, hub low on the 15T cog on the cassette to hug high, you'd probably want to shift to about 25T cog so the jump was a normal step between gears. Typically that will be three or four cogs.

The Schlumpf Mountain Drive gives a 2.5x gear down in the bottom bracket but I'm not sure they're still manufactured. I've used one of those and found it effective but the jump in gears was huge. It was more like shifting between low ratio and normal in a 4WD vehicle than a normal bicycle gear change. Typically I'd go from bottom gear on the derailleur system to the smallest cog on the cassette when I changed, but often the bike would almost come to a halt when i did that because I was going up a hill and not pedalling hard for the 2-5 second gear change cost me all my momentum. I fear that a similar shift in the rear hub would have the same effect.