You have asked a good question that many people would probably
think but hesitate to raise.
You have to understand that merits and demerits are not like
money
.I have $100 in my karmic account, and when I donate $80 of
merits to you, I only have $20 left.
(1) In transference of merits, I get the benefits (even
without intention of gaining benefit for myself)
It is taught that when I generate positive thoughts and dedicate to
you, even though I may not have any self-centred wish of gaining
any benefits from this, by virtue of my selflessness and
generosity, I have already gained great benefits from it. Just look
at your mind, when you are able to generate even a thought of
kindness to someone and wishing someone be freed from pain and
suffering, at that instant your mind is already more open, kind and
tender than the moment before when you were thinking of ignoring
that persons suffering.
(2) When I transfer merits, my body, speech and mind become
kinder, and affects others positively
Therefore, the more kindness me and you generate and dedicate to
the person, the more compassionate our minds and behaviours become,
which in turn can affect the person whos suffering or the people
around him/her. For example, if I am someone taking care of this
sick lady, and I posted a request for people to help dedicate
merits to her. And when I come back to the website and I saw so
many people actually dedicating their best wishes to her, and I
know more many are silent but actually dedicating their merits to
her on their own, I feel more comforted, I feel rejoiced, and I
also feel more encouraged. Therefore when I take care of the sick
lady, I will be acting more positively in body (I am gentle in the
way I touch her), speech (I am kind and soothing in my words) and
mind (I am positive and kind in my thoughts). All these can affect
her in the positive ways. Whereas without these, I could be angry
and frustrated, and these can affect the way I treat her in my
body, speech and mind.
(3) If the sick person can rejoice, he/she benefits more
from it
Now if this sick lady is able to rejoice in our kind intentions,
that persons mind also generates positivity which definitely
benefits her own mindstream.
That is a simple way of explaining how transference of merits
work.
As for your question of whether the same logic can be applied to
demerits. I think the conversation between King Milinda and
Venerable Nagasena can help you. 
King Milinda raised a similar question to Ven
Nagasena
..
If some people prepared a meal and visited their relatives but
those relatives did not accept the gift, would that gift be
wasted?
No, venerable sir, the owners themselves would have it.
Just so, O King, the givers derive benefit from their
almsgiving.
Is it then possible to share demerit?
This is not a question you should ask, O King. You will be asking
me next why space is boundless and why men and birds have two legs
whilst deer have four!
I do not ask this to annoy you, but there are many people in the
world who are perverted or who lack common sense.
Though it is possible to ripen a crop with water from a tank it is
not possible to use seawater. An evil deed cannot be shared
with one who has not done it and has not consented to it.
People convey water long distances by means of an aqueduct but they
cannot convey solid rock in the same way. Unwholesomeness is a mean
thing but wholesomeness is great.
Give me an illustration.
If a tiny drop of water were to fall on the ground would it flow
over ten or twelve leagues?
Certainly not, it would only affect the spot where it fell.
Why is that?
Because of its minuteness.
Just so, O King, unwholesomeness is a mean thing and
because of its minuteness affects only the doer and cannot be
shared. However, if there was a mighty cloudburst would
the water spread around?
Certainly, venerable sir, even for ten or twelve leagues.
Just so, O King, wholesomeness is great and by reason of
its abundance can be shared by gods and men.
Ven Nagasena, why is it that unwholesomeness is so limited and
wholesomeness so much more far-reaching?
Whoever, O King, gives gifts, observes the precepts and performs
the Uposatha, he is glad and at peace, and being peaceful his
goodness grows even more abundantly. Like a deep pool of
water from which as soon as water flows away on one side it is
replenished from all around. Just so, O King, if a man were to
transfer to others the merit of any good he had done even for a
hundred years the more would his goodness grow. This is
why wholesomeness is so great. However, on doing evil, O King, a
man becomes filled with remorse and his mind cannot escape from the
thought of it, he is depressed and obtains no peace, miserable and
despairing he wastes away. Just, O King, as a drop of water
falling onto a dry river-bed gains not in volume but is swallowed
up on the very spot where it fell. This is why
unwholesomeness is so mean and minute.
