I believe the first short story is "Mind Partner" by Chris Anvil. You can obtain a copy from Baen books at:
Baen Books - The Trouble with Aliens
Points that match are:
One of the stories was about an undercover agent (in the future I guess) who poses as a customer (for a most dangerous drug) to infiltrate the criminal network which distributes or produces this drug.
Protagonist goes undercover to find drug
After he is injected with the drug by the nurse or one of the drug people, police raid the building and capture the criminals. The agent has a happy life after that until his 80th birthday when he wakes up and find himself that he is still in bed where he got the drug (that is, whatever he saw until 80th birthday was a dream), the police raid and this time the criminals get away.
In the first life he lives as a Private investigator, works with his son, etc. In a second life he dies destitute, in a third he is a painter, etc. The interesting thing is that he remembers every life with perfect detail.
Again so many years later he wakes up to find himself in the same place (the room where he got the drug). This keeps going on (his life is a continuous waking up from dreams) as the result of taking the drug
we eventually learn that it is not actually a drug that is causing the dreams, but an alien that works as a "Mind Partner" to help humans experience such lives.
This was anthologized in a number of locations. I have listed some below. You might recognize the other story from one of them.
The Trouble With Aliens (2006): Reprints the "War With the Outs" stories along with three short stories retroactively rewritten to fit the in-universe chronology and a new conclusion to the sequence. The volume also includes other stories on theme of human contact with aliens, from the human perspective.
Includes "The Prisoner", "Seller's Market", "Top Rung", "Symbols", "Foghead", "The Ghost Fleet", "Cargo For Colony 6", "Achilles's Heel", "Of Enemies and Allies", "The Kindly Invasion", "Mission of Ignorance", "Brain's Isn't Everything", "The Captive Djinn", "The Uninvited Guest", "Sabotage", "Mind Partner", "A Question of Identity" and "Advance Agent".
Index to Science Fiction Anthologies and Collections
Mind Partner and 8 Other Novelets from Galaxy ed. Horace L. Gold (Doubleday, 1961, hc)
◦Mind Partner · Christopher Anvil · nv Galaxy Aug 1960
◦The Lady Who Sailed the Soul · Cordwainer Smith · nv Galaxy Apr 1960
◦The Stentorii Luggage · Neal Barrett, Jr. · nv Galaxy Oct 1960
◦Snuffles · R. A. Lafferty · nv Galaxy Dec 1960
◦The Sly Bungerhop · William Morrison · nv Galaxy Sep 1957
◦Blacksword · Andrew J. Offutt · nv Galaxy Dec 1959
◦The Civilization Game · Clifford D. Simak · nv Galaxy Nov 1958
◦The Hardest Bargain · Evelyn E. Smith · nv Galaxy Jun 1957
◦With Redfern on Capella XII · Charles Satterfield (by Frederik Pohl) · nv Galaxy Nov 1955