Those with an active interest in divination will rarely be found focusing on just one method and those who've successfully mastered one will go on to master others. So, how easy is it to mix different methods in one reading? Is it advisable? Does it offer the querent more clarity or just serve to muddy the waters?
The first piece of advice I'd offer is to practice dual method readings extensively before trying them out on willing querents. If you've mastered both Tarot and Runes, for example, you may feel confident enough about their meanings and messages to be capable of using them together without any practice but then find yourself flustered and confused in front of a querent when you realise that it's not as easy as it seems. Each method creates a different kind of atmosphere. Different methods offer varying degrees of information. For example, scrying can offer images and scenes that may need extensive interpretation by the reader, whereas Tarot can offer notably specific practical advice. Many people use different methods dependent upon the type of reading they want to give, perhaps using palmistry to look at who the querent is as a person but then choosing hydromancy to focus on what's in store for them over the coming year. Different methods can fit together harmoniously if you've managed to establish what they mean to you first. I personally choose to combine Runes and Tarot occasionally. For me, the Runes do not offer as much practical advice or specifics, but what they do offer very well are key points for spiritual concern and forecasts of what will be important to the querent at different stages. They're useful in fleshing out a reading and also to confirm the card messages if I need clarification myself.
Before attempting dual method readings it might be useful to jot down a few of these questions and answer them in order to gather ideas and establish how to do it effectively.
Different methods for different focus areas?
Do you want to use Tarot for practical concerns and Runes for spiritual concerns? Palmistry for the past and I Ching for the future? Which of your methods do you feel is most suited to questions about romance and love? Which method would you choose for a question about money issues?
Equal use or minor/major?
Do you want both methods to feature equally in the reading? Or is there a central method and then a lesser method to punctuate the main messages? Do you think that giving equal strength to the messages from both methods will be confusing? Is it best to see what each method brings up before deciding which one holds sway?
What are your reasons for using two methods?
Do the methods compliment each other? Are you equally comfortable with both methods? Are you using dual methods for the right reasons (ie to give more effective readings, not just to impress the querent or get more practice)?
Some readers like to offer their querents a choice of methods. This can also be effective, if you take some time to explain what the different methods tend to offer. For example, explain to the querent that your palmistry readings tend to be more future based, instructive and down-to-earth whereas Runes tend to lead the reading in a much more spiritual direction that deals mainly with past events. That can give the querent more control over what kind of help they receive. Equally, it's acceptable to listen to the querent's area of focus before selecting a method you feel will be most helpful. You might feel that love and romance questions are best addressed using Runes, for instance..
Some Pagans believe it's useful to use the divinatory method with the closest ancestral significance for the querent. Whilst I don't necessarily feel it's important for Chinese querents to be given I Ching readings and those with Celtic roots to be given Rune readings, the main thing is that the belief in the value of that technique will make it valuable to that reader, and so give the readings more power.
Only you can really be sure if dual readings work for you as a reader. If you regularly feel that the Runes you're drawing do not tie in with the Tarot cards or confuse the messages, it's obviously not beneficial to use both together. But practice in this area will confirm the success rate before you use it on querents.