Planets do not stand independently in Rashis or Signs.  One planet is always connected to another planet or two or more planets some way or other- by association, Karakatwa or Rashi lordship. 

Apart from the planetary inter-connections, we find two-planet combinations occur in most Natal Charts. 

Synergy theory 
Some astrologers who are also conversant with modern science apply the ‘synergy’ theory to interpret the effects of planetary combinations.  ‘Synergy’ is the extra power or energy that is generated when two or more elements or forces are combined or blended.  They say that two different metals separately may not be as hard and strong as their alloy. 

Let us take for example, the Sun –Mars conjunction in a sign.  Sun, represents, among other things, activity and commanding power and authority.  Mars represents vigour, physical strength and aggressive nature. Sun- Mars’s conjunction represents a blend of these characteristics.   The synergic effects of this combination can be either negative or positive depending on certain modifying factors, the nature the House and the sign where the conjunction occurs, malefics or benefic aspects it receives, Karakatwa of the Houses and the two planets. 

On the positive side, this conjunction can make the native strong in physique and spirit and also proactive.  On the negative side, this conjunction can make the native a rash person going into action in an aggressive manner without foresight or forethought. 

Sun, represents, among other things, activity and commanding power and authority.  Mars represents vigour, physical strength and aggressive nature

Dwi-graha Samyoga in more senses
The meaning of two-planet combination need not be restricted to construing that two planets are in conjunction in the same sign. The term combination can also be used to mean one planet is parallel, semi-square, sextile, square, trine or opposition to another. 

Aspects
When a planet is aspected by another planet, the aspected planet and the aspecting planet too form a combination producing such specific results as in a Dwi-graha Samyoga. 

Transits
The meaning of planetary combination can be extended to transits as well.  For example, Sun transiting over the Natal Mars can result in a two-planet combination.  However, in interpreting results you have to take into consideration the astrological factors that modify the results produced by such a combination.

Planets ruling Dasas
The concept of two-planet combination can be applied to cover the lords of Dasa and Antar Dasa or Videsa – Sukshama Dasa combinations under the Vimshottara Dasa system as well.  Take for example, Mars’ Antar Dasa in the Maha Dasa of Sun.  During this period, the native may become more energetic and proactive for self-advancement on the positive side or become bellicose and quarrelsome on the negative side. 

Planets in signs
A two-planet conjunction situation can arise when a planet occupies a sign ruled by another planet.  For example, take the case of Jupiter occupying Aries. This is too a typical two-planet conjunction: Jupiter and Mars producing the effects specific to Jupiter- Mars’s combination.