hello,
i have a server running centos on a adsl 24mbps down/1mbps up with a static ip
the problem is that now and then i'm facing downtimes
and i thought to get a 2nd line as a backup
but how am i going to merge these 2 lines
they will be from different providers with different ips
my think is to use 2 nics, 1 for each line
and use different IN A records on the domain's zone file
but again if one of these lines is out 50% of people trying to use that site won't be able to
any ideas?

You can try a dual wan firewall for one. Look here.

this will work ok if i want the server to access the internet
but what happens when i want internet to access the server
2 lines means 2 ip addresses
if one of them is down people who try to access a website that directs to that ip address will fail
my thought was to use 2 IN A records in the domain zone file, 1 for each address
this will give a 50% chance of fail
but still it's not tnough

c'mon guys i thought there were a lot of experts in networking in here
you helped me out various times
can't you help me with this one
i've been stretching my mind trying to figure out what to do

You're right. But why they have to use imagination.

Tell, what kind of downtimes your server has?
Where are they happen? Are these ISP problems or your equipment?

isp related problems
i live in greece and all isps have same issues
so my think is to combine 2 different isps
they can't be down at the same time

If you connect to two different ISP, you can't get the same IP.

The only idea is to talk with both of them in order to get better service, and choose one.
You can pay more to one of them, and may be get better service.

Or you can try to use Dynamic DNS, so when one will go down, and you will get different IP, server can announce it.

i, see
so there is no possibility to have bind "know" when one of 2 ips is down and to send all dns requests to the other ip?

If you google Dynamic DNS you will find Companies that will host your URL and dynamically point toward the changing IP address. So the IP address changes but the url is static

yes i'm aware of dyndns.org as i have used their services in the past
but i can't use my own nameservers with dynamic dns or am i wrong?

about 5 mins ago i had a shower and i had a very crazy idea which i think it will work
but i really need your opinion
well, here it is:

eth0 is one provider
eth1 is the 2nd provider

i will use 2 dns servers(bind) one for eth0 and one for eth1
ns1.mydomain.tld will go to ip of eth0 and ns2.mydomain.tld to ip of eth1

now the first dns server will have A record for ip of eth0
and the 2nd ndns server will have A record for ip of eth1

if i set a TTL of 60secs whichever goes down will give a maximum downtime of 60secs
am i right?
will it work?