Beginners to 5e rules are well served starting at first level
First off, since everyone is brand new, I would advise against starting at anything beyond level 1. The Dungeon Master’s Guide advises: “When the new player is complete unfamiliar with the D&D game…have that player start with a 1st-level character.” (DM Guide, New Players, p. 236)
D&D 5e lets you delay a lot of weighty choices about your character until levels 2 or 3 (fighting style, college of magic, paladin vow, etc). And although having so few hit points is nerve-racking, everyone will have fewer rules to keep track of with first level characters.
You can still level up the characters a little quicker than you might otherwise.
Add an NPC Companion (so there are at least 3 party members)
Adjusting difficutly is not the only thing to do to help your players out. I’d suggest you create an NPC (of, if you like, a GMPC) character to fill out the party. After your friends make their characters, make one that will provide skills and powers the others don’t have.
This does more than add more strength to the party — the third party member will give the party more flexibility to use combat tactics. Consider, if your friends make a fighter and a wizard, the battle plan is going to be predictable: the fighter protects the wizard. And if one character goes down, the other must save him.
With a third character, there will be more possibilities, more choices, more tactical opportunities.
There are some challenges with GMPC’s, but to start out, just make sure that character doesn’t upstage your friends’ characters too often.
Other ways to increase size of the party
NPC companions don't need to be PC-style characters from the Players Handbook. You could also use “monster” style creatures, that is, a creature with a Challenge Rating. Such creatures may compete for the lime light less with player characters. Creatures with Challenge Ratings tend to have have higher hit points but do a little less damage per round that Player Characters, so they can “stand there and take it” in a fight.
This would give you a lot of latitude in what color the party would have. The companions could be Man at Arms body guards like used to be common in 80’s D&D. Or they could be animal companions, magical creatures - whatever fits the color you and the players want.
The downside to this would be the creatures don't level up to keep pace with the PC’s. On the other hand, they provide more relative advantage at the lowest levels, when it’s needed most.
One other option would be simply to let the players create and control 2 player characters apiece. This could solve all your problems at once, but not everybody enjoys juggling multiple characters. Especially not novice players.
(See *DM Guide, Small Groups, also on page 236, for more on this.)
Goblin Arrows: Your First Adventures
Even with three characters, you will want to nerf the encounters a bit. I ran the beginning of Phandelver with three experienced players. It was tough even for them.
To nerf encounters in Goblin Arrows:
For the outdoor encounters in the beginning of the module, remove goblins or wolves to get the challenge rating down to from half to three quarters of what is listed in the module.
Have the characters level up when they reach the entrance to Cragmaw Hideout.
Even with three characters and nerfed encounters, be aware the dice treat small parties unfairly more often than they mistreat larger parties. At low levels, just a few lucky rolls can knock out a character or two and put the party in a serious pickle.
The thing that helps best with this is to give your party more resources to fall back on in a pinch. Expendable items like healing potions and spell scrolls work well.
Of course, as DM, you are within your rights to flat-out cheat on the players’ behalf, if the dice are being uncooperative. “Another 20? I don’t think I rolled that right, let me try again.”
The James Bond Rule
Apply the James Bond rule: the villains always take the party alive and almost always leave the party unguarded with means of escape.
Unlikely Resting Places & Participation Awards
Probably the easiest way to dial back the difficulty in D&D 5e is to let the party get more rests. So, let your players rest in an empty room even if there are monsters nearby.
Another little advantage you can give your players is to frequently award Inspiration. You may need to remind them to use it.
Recruit another player or two
You’ve probably asked around already, but after the game begins, you may be able to find more people to play. (Some folks are more the joiner type than the founding member type.) One or two more players would make for an ideal table for beginners.