You should be able to use N number of 12 word seeds and combine them using the hex representations to give you a single 12 word actual wallet seed. The only thing you need for this is a way to generate 12 word seeds and sum them.

In my github misc repo I have a collection of seed utilities that allow you to do this and more.

Like this,

for x in 1 2; do ./seed; done | ./addseeds

seed is my util for generating a seed (it uses code extracted from Electrum)
addseeds will read stdin and sum the seeds (whether hex or 12-words) and output sum

The line above generates and prints 2 seeds and then the sum seed.

You can use the same addseeds to re-combine later like this,

echo -e "first 12 words\n2nd 12 words" | ./addseeds

or can read from a file,

cat myseeds.txt |./addseeds

(prints both inputs and output sum)

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I would also note I have a simple util there called b2b that converts bases using alphabets for each base. With this you can roll dice and output hex values to create seeds using hexseeds. eg. with dice rolls (need something like 99 rolls, not just a few as here), base 6 to hex,

./b2b 1624351  123456  0123456789abcdef