Training 2 players from 18 is easiest from D3 for a number of reasons, primarily because if you are training a player for the NT, you want to focus on secondaries first. This means training a player out of position which sacrifices tactics in favour of training for your games. To do that using 48 min sfdc 5 player lineups for 2 to 3 seasons is much easier from D3 and you have no salary pressures or table position worries plus financially given you are not having to support other players you can still make more than enough money to cover the purchase and maintenance of a level 6 trainer.
Doing all the secondaries as early as possible is not a good plan as some can be done in the later years using careful planning. For example you can usually include 3 weeks of secondaries in your training plans per season without much tactical sacrifice.
I would aim to promote between season 3 and season 4 from D3 to D2 and I would advise a 6-7 season training plan from purchase in order to plan this. In season 3-4 your training plan should include primaries training with a flexible two position training (such as one on one, JS or rebounding) for the last half of the season to allow you to play the players in 2 positions making the most of promotion opportunities. You should aim to promote to EBBL in a season where your secondaries are well rounded and you are ready to pump the primaries. If you are looking at salaries in the region of 140k+ after the season update, that is the season you should push to promote. It is a risk, you risk losing your player, but the payoff in the playoffs (no rhyme intended) is that you are using a 100k salary player who is actually a 140k salary player; who needs to buy the playoffs now?
My team for example, I had an 8 season training plan when I purchased Innes at 18 which I am still following. Mac played at PG (trust me that was painful as his secondaries used to be terrible! 3/1/1/2/1/1 if I remember right.) I chose to promote when Innes hit 21 and Mac hit 20 as this was a salary efficient time to move from D3 and allowed me to move into D2 with Mac hitting 100k salary and Innes at 50k. I then chose to do more secondary / primary work holding mid to top table. Saving the one on one and rebounding training helped here as I could use Innes at SF and Mac at PF (as they were so well rounded in secondaries).
My EBBL push year was when Innes was at 90k salary and Mac was at 140k. I trained mostly primaries that year using them in position and promoted with Innes at 140k salary and Mac at 200k - thats 110k's worth of player for free in the playoffs.
Given that Mac and Innes got their NT call up so young has helped me balance finances and allows me to carry a slightly heavier bill than most because the merchandising is so good.
I know many wouldn't advocate dropping to D3 because it is not necessary. Of course it isn't necessary, but it is the best way in my opinion. I know many would say you could train from D2 (true) and I could argue you can train from EBBL too (Muse has done it with Tansey) but that is even harder and definitely not possible to train 2 players from scratch. I know having done it, that it was a fun challenge, the risk of losing players if I didn't promote or having to sell one was huge and it allowed me to focus entirely on building 2 cracking players who are without doubt the focal point of my team and great for the NT. I am still training them, they are going to get better for at least another season and then as they become veterans.