most of the time although people refer to co-factors they aren't really talking about co-factors in the strict bio-chemistry sense
B12 and folate are used together in a number of processes in the body so not having enough one one means the process doesn't run well. It also means that if one has been deficient and you start correcting that then you may temporarily be using more of the other whilst the processes get back to normal and damage is corrected.
More likely is that if you are B12 deficient and have an absorption problem then that may well affect your ability to absorb other vitamins and minerals leading to deficiencies in those vitamins and minerals - the symptoms you mention could be indicative, for instance of a vitamin D deficiency but could also be a number of other things.
this is a link to the EFSA (European Food Standards Agency) guidelines on recommended upper limits of supplementation - it discusses the effects of deficiency, the effects of overdosing and recommends upper limits for supplementation. It is a bit old now but there are regular reviews and report that you can find on the same website
really suggest that you speak to pharmacist/gp/midwife before supplementing.
B12 levels can be lowered by pregnancy - if you don't have an absorption problem then I'd stick with folic acid and B12 until you have talked things through with a medical professional.
as I tried to explain above - co-factors are mentioned on this forum but they aren't really co-factors - the only real crossover is the relationship between B12, B9 (and to a less extent B6).
If you have an absorption problem and it is affecting your ability to break down food to extract other vitamins and minerals then it may not be possible to rely upon diet to provide those vitamins and minerals (though generally the effect isn't as dramatic as it is with B12). I do agree that if you can get all the vitamins and minerals you need from food then that is the best way of doing it.
It is true that the main link is between B12 and B9 and to a less extent B6.
I have found a few interesting information in a paper.
"B complex vitamins and vitamin C are required for the synthesis of amino acids, biogenic amines, neurotransmitters and steroids. Specifically within the CNS, the metabolism of dopamine and noradrenaline requires vitamin B2, vitamin B6, vitamin B12, nicotinamide, folate and vitamin C. Synergy between vitamin C and B vitamins also occurs in the breakdown of histamine and tryptophan in the brain (Table 2)."
"Because of their metabolic interdependence, the B complex vitamins have to be regarded as a functional unit whose individual members act like links in a chain of biochemical reactions. Within intermediate metabolism, transformations exist which require all eight B vitamins as cofactors for the various enzymes. Many interactions are also known between the different members of the complex. Although vitamin C is essential for collagen synthesis in the skin, it is noteworthy that the highest concentrations of this vitamin are found in brain tissue. Brain vitamin C is known to interact synergistically with B complex vitamins in the maintenance of several aspects of cognitive function and performance.
The minerals calcium, magnesium and
zinc are required as cofactors for numerous vitamin-dependent enzymes and also play a direct and crucial role in membrane excitability and neurotransmission."
Look up Pernicious amiena/b12 deficiency support group on fb. Get the book could it be b12? By Sally Pacholok. This a documentary on b12 and case studies. Sally is in it herself. Check out her film on YouTube. The fb group really helped me.
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Questions regarding cofactors
Where can I buy safe Methyl B12 for UK use? 
Keeping the balance of vitamines
Lightheadedness - B12 symptom
