I have heard that Rama is mantra of two Mantras. Ra from
Om namo naRayana and Ma from
Om NaMah Shivay. Is this true?
Yes it is. Ra is agni beeja and Ma is amrita beeja. It was Sage Vasishtha who christened Rama, just as Sage Garga christened Krishna. Actions performed by sages of their calibre have multiple obvious and esoteric (mantra-related) import.
Meanings of Rama's name are discussed in rAma tApanyOpanishad.
"Râmeti dvyaksharo mantrah panchâshtâksharasâratah | sarvâdhikârasiddhyartham vihitah kalitârakah || 6 ||"
The two-lettered mantra "Rama" is the essence of the panchakshari (i.e. namah sivAya) and the ashtashakri (Om namO nArAyaNAya). It bestows all gains and helps surmount the travails of kali.
Yes, it is true. It is clearly mentioned in fifth chapter of Rama Rahasya Upanishad.
Sanaka and other ascetics asked Hanuman: “ Tell us the meaning of Sri Rama mantra”.
Hanuman replied: “Among the Rama mantras the 6-lettered is the king among them all; though as mentioned earlier they exist as one-, or two-, or three, or four, or five syllables or even as six-, or seven-, or eight, or even as many more syllables. Lord Shiva knows in essence the glory of the six-syllables (mantra): The true meaning of the king among the Rama mantras, as well as of the eight-lettered Narayana and five-lettered Shiva mantras, is spoken of as thus: Where yogi-s revel in the 2 syllable mantra viz. ‘Ra ma’ where the ra syllable denotes fire wherein abides illumination.