I think that the way GW and BL writers have thought it out over time is that yes, during surgery 2 glands are put in and mature to produce future geneseed. But to me this doesn't make mathematical sense. Space Marines die at too high of a rate at their limited numbers for this to make sense. Yes an apothecary is supposed to recover it, but how many times have you read one of the following scenarios
-The Marine's Corpse is left behind
-The Marine's Corpse is ejected into space
-The Marine's Corpse is torched by a flamer
-The Marine's Corpse is reduced to slag by plasma
-The Marine's Corpse is vaporized by a melta
-The Marine's Corpse is blown up
-The Marine's Corpse is eaten by daemons/nids/orks
-The Marine's Corpse is shot up way too much
-The Marine's Corpse is hit by a Gauss weapon
-The Marine's Corpse on a ship when it goes KABOOM
Just to name a few. And really, the above are mostly the only way to kill a space marine short of removing his head.
So the issue with the above deaths? Well the apothecary can't recover the gene seed. So if two are required to make the next generation of Space Marine then that is one less Space Marine that can be made. Granted there are depositories of geneseed, but unless every chapter has like 50 of the fortresses sitting around that exist in the novel Storm of Iron, then over 10,000 years, at the rate Space Marines die in the above fashion, then each chapter would run out of gene seed.
So what are the alternatives? Well...
1- There is no alternative. Simply accept the way I first stated and assume that GW and BL employees and writers have a poor understanding of fundamental mathematics. A diminishing finite amount of geneseed will eventually see SM chapters die off.
2- One progenoid is implanted as the gene seed in a neophyte and his developing body grows a second one as a result. This would allow the numbers to grow slowly over time as it is suggested for recovering chapters, but slow enough that when a chapter falls below a critical threshold, they may die at a rate faster than they create. Examples of scenarios like this could be the Flesh Tearers and Celestial Lions.
4- Geneseed is removed not only at death, but from all Space Marines at some point. Basically, say once it has been in you for 50-100 years, they just take it out to implant in another. This seems the least likely really, as I have never seen it mentioned. Plus, though more delayed than option 1, it would eventually fall to the same fate. A fixed number of geneseed would ultimately see SM chapters disappear. Though it does raise the question, do apothecaries remove the progenoids before placing a brother in the sarcophagus of a dreadnought. I would hope so, otherwise it seems a waste especially since most dreadnought seem to end their life going up in a puff of smoke (and whoosh of fireball), immolating the pilot instantly.