[velo99]

Second bike is for what purpose?
A spare or different riding applications?
From my point of view, keep your old bike as a spare & ride the new one. Replace / upgrade worn or badly underperforming parts on the old bike.

 

[hankscorpio]

X+1 = the number of bikes you need

 

[AndrwSwitch]

All of the above.

I still have my old hardtail. I also have a 'cross bike, a road bike with discs I used to commute on, a track bike, and my shiny new FS.

I did get rid of a road bike about a year ago and the track bike's on thin ice. But I use all the others pretty frequently.

 

[bmreal]

Exactly.
My other bikes are there as spares or for different applications, and of course the N+1 thing.

If I need work done on one bike, I have another to ride in the mean time. I have a rigid SS set up for commuting, another rigid SS as my main trail bike, a 1x10 HT I use for new or unfamiliar trails and an old steel 26er for riding around the hood with the kids. I haven't really found a good excuse (or all the $$) to build a FS, but I just got a new gig at my LBS so that may be changing soon.