You can safely eat those two sandwiches.
Also the darker tuna is safer than the all white one.
Mercury has been linked to brain defects and also to possibly higher levels of ADD/ADHD though I suspect those studies are inthe very early stages so far.
Larger game type fish have higher levels of mercury.
It's all about moderation. Fish is good for you with all the Omega3's and protein it's still a brain food. You just have to be aware of what types you eat and to not consume too much mercury containing ones.
From the FSA
http://www.eatwell.gov.uk/agesandstages/pregnancy/whenyrpregnant/"Some types of fish
You can eat most types of fish when you’re pregnant. But there are a few types you should avoid and some others where you should limit the amount you eat.
Avoid eating any shark, swordfish and marlin. Limit the amount of tuna you eat to no more than two tuna steaks a week (weighing about 140g cooked or 170g raw) or four medium-size cans of tuna a week (with a drained weight of about 140g per can). This is because of the levels of mercury in these fish. At high levels, mercury can harm a baby's developing nervous system.
Have no more than two portions of oily fish a week. Oily fish includes fresh tuna (not canned tuna, which does not count as oily fish), mackerel, sardines and trout.
But remember that eating fish is good for your health and the development of your baby, so you should still aim to eat at least two portions of fish a week, including one portion of oily fish"