rman backup rac

i read aticle doc id:243760.1 

backup rac database,

run {
allocate channel d1 type disk connect 'sys/rac@node1';
allocate channel d2 type disk connect 'sys/rac@node2';
  set until sequence 14 thread 2;
   restore controlfile;
  alter database mount;
release channel d1;
release channel d2;

there is a question, why we use two channel

such 'sys/rac@node1' 'sys/rac@node2'as  backup database, can we use only one channel to backup database?

can we use  command such as  "allocate channel d2 type disk connect 'sys/rac@node1 as sysdba' to backup database"?

is there  any difference between backup rac and backup single database?