Taking sample dates of Wed 15 Feb out and Wed 22 Feb back, I'm seeing a £30 outbound fare on the Virgin Trains service from Euston to Edinburgh at 0643. After that there are plenty of Virgin Trains East Coast services at £40 departing from Kings Cross. (e.g. at 0700, 0730, 0900, 0930, 1000, 1030, etc). There's also another £30 service from Euston at 1043. (Note that the route from Euston takes about an longer than the one from Kings Cross though).
Coming back there are also some £30 fares from Edinburgh to Euston (e.g. at 0652 and 1051). Otherwise there are plenty of services to Kings Cross available for £40. (e.g. 0655, 0830, 0900, 0930, 1000, etc).
I doubt that ticket splitting would produce any lower fares (and a quick check doesn't seem to find any), so £60 return, per person, is probably the lowest you'll find between London and Edinburgh (unless you use a Railcard though - see below).
Using the same dates for travel between London Euston and Glasgow finds a very small number of trains priced at £30 (e.g. 1230 ex Euston and 0940 ex Glasgow) but rather more at £40 (although your options are still quite limited for the outbound journey).
Again, ticket-splitting doesn't seem to help.
If you were to buy Senior Railcards you need one each, costing £30 per year per person (available from stations or online) or £70 per person for three years (available online only). That would give you a discount of 34%, taking the £30 fares down to £19.80, thus giving a return fare of £39.60 per person.
There's clearly no point buying Senior Railcards though if you don't anticipate any future rail travel. However they might be worth considering if you'll be using other train services at some time. (There are no time restrictions with Senior Railcards, other than for travel wholly within London and the Southeast, and it wouldn't matter if any future travel was together or alone).
Alternatively, as NJ has mentioned, you can buy a Two Together railcard, which is only valid after 0930 (except at weekends and on public holidays) and can only be used when
both of the people named on it are travelling together. That would be cheaper as you'd need only one card, costing £30 for a year. (There's no three-year option with that card).
So, as I see it, your best option is probably to buy a Two Together Railcard for £30, then look for fares discounted down to £19.80 each way, giving a total cost (for two people) of £30 + (2 x 2 x £19.80) = £30 + £79.20 = £109.20.
The ONLY place to search for fares is here:
http://www.nationalrail.co.uk/
(Do NOT use third-party sites, such as The Trainline. They can't find anything cheaper and actually work out DEARER because of their booking fees. They've also been known to issue tickets which aren't even valid for travel on any trains!).
If you require information about Senior Railcards (because you anticipate that you'll be travelling separately, or before 0930 weekdays, in future), see here:
http://www.senior-railcard.co.uk
For information about the Two Together card, see here:
https://www.twotogether-railcard.co.uk
To see how far ahead you can buy Advance tickets, see here:
http://www.nationalrail.co.uk/times_fares/ticket_types/44703.aspx
(Note that you need to refer to 'Virgin Trains' for services from Euston but 'Virgin Trains East Coast' for services from Kings Cross).
I reckon that's about as far as I can go with the limited information you've provided. If you still need assistance after you've decided on your dates and your actual destination, please post again.