NO.
In both Vedic and Agamic Dharmas, a Guru has to be someone who is alive. I too have some friend who consider Adi Shankara as his Guru. But unless he takes DikshA from a living Guru he will be technically a uninitiate (adikshita) only.
Guru or Teacher in the Vedic Dharma:
The first birth is from the mother and the second at the tying of the Muñja-grass girdle. At the latter birth, the Savitri verse is his mother, while the teacher is said to be his father.
The man who initiates a person and teaches him the entire Veda is called ‘teacher’; whereas someone who teaches just a section of the Veda is called ‘tutor’, as also someone who teaches the Vedic Supplements.
VDS 3.20
So, obviously, the teacher or Guru has to be alive to initiate the student and teach him (the disciple) the Vedas.
The student, after being initiated, has a lot of duties and he has to stay along with his teacher in his (the teacher's) house.
Student––he should serve his teacher until death and, when the teacher dies, the sacred fire; for it is stated, ‘The fire is your teacher’
12 He should wait upon his teacher, walking behind him if he is walking, standing by if he is seated, and sitting if he is lying down. 13 He should recite his lesson only when he is called upon to do so. 14He should announce everything he has received to the teacher and eat with his permission.
VDS verses.
So, it is more than obvious that we are talking about a living Guru here.
Guru in the Agamic Dharma:
In Agamic DikshA SamskAra too, a Guru has to living in order transfer energy, consciousness etc onto the disciple. The following verse defines who is the Guru precisely.
EkAkshmarapradAtAram yo gurunchA vamanayet |
SwAnam yoni satam gatvA chandAlatvam vApnuyAt ||....
One who insults the Guru, who is the giver of ekAkshara (or OM), gets the womb of dogs for hundred births and thereafter gets chandAla-hood (low human birth).
KT 11.74.
So, here the Guru is defined to be the one who gives disciple the ekAkshara or the OM. Now, obviously doing so is not possible for someone who is dead.
DikshA is actually a process where the Guru gives mantra into Shisyas ears a particular number of times. Their nearness (sAmipya) is essential in the process. Needless to say both have to alive to participate.
The next verse says, that all kinds of knowledge, conducts, scriptures, mantras are successful only if they are learned from Guru's mouth. And, there is no other way.
PAramparyA AgamAmnAyAm mantrAchArAdikam priye |
Sarvam guru mukhAllabdham saphalam syAnna chAnyathA ||............
ParmaparA (lineage), Agamas, AmnAya, Mantras, AchAra (conducts) are all fruitful if obtained from Guru's mouth ( i.e directly learned from a personal living Guru), and otherwise not.
KT 11.46.
Further, it is stated in scriptures that just like a wife's sins get attached to her husband, likewise a Guru has to pay for his disciple's misdeeds. Hence, Guru, before initiating the disciple, has to test him in many ways. In fact, they both have to mutually test each other before the DikshA.
The following verse gives the amount time for which such tests are to be conducted:
EkAvedana dvijo yogyah kshatriyo vatsara dvAt |
Vaishyo yogya stribhirvashairshchaturbhih shudra eva cha ||...........
The Brahmin becomes competent [for dikshA] after one year, a Kshatriya after two years, a Vaishya after three years and a Shudra after four years.
KT 14.105.
EkAvedana in the verse means one year. So, for a Brahmin disciple, the mutual test time is one year, for Kshatriya it's two years.... and for a Shudra disciple it's four years.
Now, obviously Guru who is alive can do such tests and can be the subject of such tests.
Can I have multiple gurus (Guru Tulya) unless I get a guru where I get personal attention, which is not possible these days?
Well, such persons have not actually become your Gurus. So, there is no harm in mentally accepting more than one such Saints as your mentor or protector.
Anyways, even in case of actual Gurus, more than one Gurus is allowed.
MadhuluvedhA yathA bhringah pushpAt pushAntaram vrajet |
JyAna luvdha stathA shishyah guro gurvantra vrajet ||.........
Just like the bee desirous of honey moves from one flower to the other, a disciple desirous of know;edge can likewise move from one Guru to another.
KT 13.132
And, similar thing has been stated in the Uddava Gita too.
Abbreviations used:
KT- KulArnava Tantram ( All verses in this text are spoken by Lord Shiva to his consort Sri Devi).
VDS- The Dharma Sutras of Vashishta.